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Summary of Significant Accounting Policies - Contract Assets and Liabilitiess (Details) - USD ($)
$ in Thousands
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Accounting Policies [Abstract]      
Contract assets $ 453,832 $ 601,268 $ 576,891
Contract liabilities $ 528,864 $ 606,146 $ 425,961