Quarterly report pursuant to Section 13 or 15(d)

Stockholders' Equity (Details)

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Stockholders' Equity (Details) - USD ($)
1 Months Ended 3 Months Ended 6 Months Ended 12 Months Ended
May 29, 2018
May 31, 2014
Jun. 30, 2018
Jun. 30, 2017
Jun. 30, 2018
Jun. 30, 2017
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Jun. 05, 2018
Jun. 04, 2018
Class of Stock [Line Items]                      
Proceeds from public offering, net of underwriters’ discount and offering costs         $ 441,799,000 $ 0          
Offering Costs $ 17,100,000                    
Common stock, shares authorized (in shares)     250,000,000   250,000,000   125,000,000     250,000,000 125,000,000
Authorized amount of common stock repurchase   $ 50,000,000                  
Number of shares repurchased (in shares)     0 0 0 0          
Common Stock                      
Class of Stock [Line Items]                      
Repurchase period (in years)   1 year         1 year 1 year 1 year    
Public Stock Offering                      
Class of Stock [Line Items]                      
Number of shares issued in underwritten common stock offering (in shares) 8,050,000                    
Proceeds from public offering, net of underwriters’ discount and offering costs $ 458,900,000                    
Proceeds from public offering, net of underwriters’ discount and offering costs $ 441,800,000                    
Over-Allotment Option                      
Class of Stock [Line Items]                      
Number of shares issued in underwritten common stock offering (in shares) 1,050,000                    
Sale of stock price (in dollars per share) $ 57