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Basis of Presentation and Summary of Significant Accounting Policies - Carrying Values and Fair Values of Debt (Details) - USD ($)
$ in Thousands
Jun. 30, 2019
Dec. 31, 2018
Reported Value Measurement    
Debt Instrument [Line Items]    
Debt instruments, excluding capital lease obligations $ 9,729,534 $ 9,952,385
Estimate of Fair Value Measurement    
Debt Instrument [Line Items]    
Debt instruments, excluding capital lease obligations $ 10,055,474 $ 9,828,274