Annual report pursuant to Section 13 and 15(d)

Stock-based Expense - Summary of Stock Option Transactions Under Equity Plan, Stock Incentive Plan, Multifamily Technology Solutions Plan and Board Plan (Detail)

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Stock-based Expense - Summary of Stock Option Transactions Under Equity Plan, Stock Incentive Plan, Multifamily Technology Solutions Plan and Board Plan (Detail) - $ / shares
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Number of Shares      
Number of Shares, Beginning Balance 3,607,089 5,801,873 5,566,888
Number of Shares, Granted     2,434,198
Number of Shares, Exercised (1,344,569) (1,568,699) (809,303)
Number of Shares, Forfeited/cancelled (61,892) (625,431) (1,389,910)
Number of Shares, Expired (163) (654)  
Number of Shares, ending balance 2,200,465 3,607,089 5,801,873
Range of Exercise Prices      
Range of exercise prices beginning balance, lower limit (in dollars per share) $ 2.55 $ 0.91 $ 0.91
Range of exercise prices beginning balance, upper limit (in dollars per share) 29.50 29.50 29.50
Range of exercise prices, Granted lower limit (in dollars per share)     18.79
Range of exercise prices, Granted upper limit (in dollars per share)     23.10
Range of exercise prices Exercised, lower limit (in dollars per share) 5.04 1.68 0.91
Range of exercise prices Exercised, upper limit (in dollars per share) 29.50 27.18 21.60
Range of exercise prices Forfeited/cancelled, lower limit (in dollars per share) 15.19 4.28 5.04
Range of exercise prices Forfeited/cancelled, upper limit (in dollars per share) 25.70 29.50 29.50
Range of exercise prices Expired, lower limit (in dollars per share) 2.55 0.91  
Range of exercise prices Expired, upper limit (in dollars per share) 2.82 0.91  
Range of exercise prices Ending Balance, lower limit (in dollars per share) 4.28 2.55 0.91
Range of exercise prices Ending Balance, upper limit (in dollars per share) 29.50 29.50 29.50
Weighted-Average Exercise Price      
Weighted average exercise price, Beginning Balance (in dollars per share) 19.58 19.43 18.89
Weighted average exercise price, Granted (in dollars per share)     19.81
Weighted average exercise price, Exercised (in dollars per share) 20.09 18.16 14.97
Weighted average exercise price, Forfeited/cancelled (in dollars per share) 19.66 21.77 20.54
Weighted average exercise price, Expired (in dollars per share) 2.73 0.91  
Weighted average exercise price, Ending Balance (in dollars per share) $ 19.26 $ 19.58 $ 19.43