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Subsequent Events (Details) - USD ($)
$ / shares in Units, $ in Thousands
 1 Months Ended 12 Months Ended
Feb. 21, 2018
Feb. 15, 2018
Jul. 18, 2017
Jul. 31, 2017
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Subsequent Events              
Number of common stock shares issued from underwriting agreement       21,122,449      
Price per share of common stock issued (in dollars per share)       $ 12.25      
Gross proceeds from issue of common stock before deducting underwriting discounts and commissions       $ 258,800 $ 243,037   $ 243,042
Net proceeds from the issuance of common stock after deducting underwriting discounts and commissions     $ 243,000   $ 243,037 $ 97,068 $ 243,042
Common Stock              
Subsequent Events              
Number of common stock shares issued from underwriting agreement       21,122,449 21,122,449   19,528,302
Price per share of common stock issued (in dollars per share)       $ 12.25      
Gross proceeds from issue of common stock before deducting underwriting discounts and commissions       $ 258,800 $ 212   $ 195
Net proceeds from the issuance of common stock after deducting underwriting discounts and commissions     $ 243,000        
Subsequent event | Common Stock              
Subsequent Events              
Number of common stock shares issued from underwriting agreement   19,354,839          
Price per share of common stock issued (in dollars per share)   $ 15.50          
Gross proceeds from issue of common stock before deducting underwriting discounts and commissions   $ 300,000          
Net proceeds from the issuance of common stock after deducting underwriting discounts and commissions $ 282,000