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Significant Accounting Policies Significant Accounting Policies (Details) - USD ($)
$ in Thousands
3 Months Ended
Mar. 31, 2018
Mar. 31, 2017
Jan. 01, 2017
New Accounting Pronouncements or Change in Accounting Principle [Line Items]      
Decrease in cash inflows from cash flows provided by operating activities $ 37,349 $ (201,039)  
Increase in cash flows used in investing activities $ 704,719 8,896  
Accounting Standards Update 2016-15      
New Accounting Pronouncements or Change in Accounting Principle [Line Items]      
Decrease in cash inflows from cash flows provided by operating activities   20,000  
Increase in cash flows used in investing activities   $ 20,000  
Retained Earnings | Cumulative effect of adoption of ASU 2016-09      
New Accounting Pronouncements or Change in Accounting Principle [Line Items]      
Cumulative effect of new accounting principle in period of adoption     $ 2,200