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Summary of Significant Accounting Policies - Cash, Cash Equivalents and Restricted Cash (Details) - USD ($)
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Dec. 31, 2014
Accounting Policies [Abstract]        
Cash and cash equivalents $ 77,671,000 $ 200,996,000    
Restricted cash 0 62,457,000    
Total cash, cash equivalents, and restricted cash shown in the consolidated statement of cash flows $ 77,671,000 $ 263,453,000 $ 97,542,000 $ 70,768,000