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Summary of Significant Accounting Policies - Contract with Customer, Asset and Liability (Details) - USD ($)
$ in Thousands
3 Months Ended
Mar. 31, 2018
Dec. 31, 2017
Accounting Policies [Abstract]    
Contract assets $ 498,190 $ 497,292
Contract liabilities 506,203 $ 433,387
Revenue recognized related to amounts in contract liabilities outstanding at the beginning of period $ 264,000