Debt - Summary of Collateral Covenants and Cross Default Provisions (Details) $ in Billions	12 Months Ended
	Dec. 31, 2018 USD ($)
Credit Agreement
Debt Instrument [Line Items]
Unrestricted liquidity required for credit agreement	$ 2.0
Minimum ratio of appraised value of collateral for Credit Agreement	160.00%
6.375% Senior Notes due 2018
Debt Instrument [Line Items]
Fixed interest rate	6.375%
Debt instrument maturity year	2018
6% Senior Notes due 2020
Debt Instrument [Line Items]
Fixed interest rate	6.00%
Debt instrument maturity year	2020
5% Senior Notes due 2024
Debt Instrument [Line Items]
Fixed interest rate	5.00%
Debt instrument maturity year	2024
4.25% Senior Notes due 2022
Debt Instrument [Line Items]
Fixed interest rate	4.25%
Debt instrument maturity year	2022