Debt - Recourse Debt (Details) - USD ($) $ in Millions	1 Months Ended	3 Months Ended		9 Months Ended
	Mar. 31, 2017	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Mar. 01, 2017
Debt Instrument [Line Items]
Gain (loss) on extinguishment of debt		$ (11)	$ (49)	$ (187)	$ (44)
7.375% Senior Notes Due 2021 [Member] [Member]
Debt Instrument [Line Items]
Redeemed notes	$ 276			690
5.5% Senior Notes Due 2024 [Member] [Member]
Debt Instrument [Line Items]
Redeemed notes		671
5.5% Senior Notes Due 2025 [Member] [Member]
Debt Instrument [Line Items]
Redeemed notes		$ 29
8.0% Senior Notes Due 2020 [Domain]
Debt Instrument [Line Items]
Redeemed notes	$ 24			$ 228	217
5.125% Senior Notes Due 2027 [Member]
Debt Instrument [Line Items]
Redeemed notes					240
Senior Notes [Member] | Convertible Debt [Member]
Debt Instrument [Line Items]
Issued senior notes			517		517
Senior Notes [Member] | LIBOR 2.00% Senior Notes Due in 2022 [Member]
Debt Instrument [Line Items]
Issued senior notes			$ 525		$ 525
Senior Notes [Member] | 4.0% Senior Notes Due 2021 [Domain] [Domain]
Debt Instrument [Line Items]
Interest rate on senior notes		4.00%		4.00%
Issued senior notes		$ 500		$ 500
Senior Notes [Member] | 4.5% Senior Notes Due 2023 [Domain] [Domain]
Debt Instrument [Line Items]
Interest rate on senior notes		4.50%		4.50%
Issued senior notes		$ 500		$ 500
Senior Notes [Member] | 5.5% Senior Notes Due 2024 [Member] [Member]
Debt Instrument [Line Items]
Interest rate on senior notes		5.50%		5.50%
Senior Notes [Member] | 5.5% Senior Notes Due 2025 [Member] [Member]
Debt Instrument [Line Items]
Interest rate on senior notes		5.50%		5.50%
Senior Notes [Member] | 8.0% Senior Notes Due 2020 [Domain]
Debt Instrument [Line Items]
Interest rate on senior notes			8.00%		8.00%
Senior Notes [Member] | 5.125% Senior Notes Due 2027 [Member]
Debt Instrument [Line Items]
Interest rate on senior notes			5.125%		5.125%
Issued senior notes			$ 500		$ 500
Unsecured Debt [Member] | 7.375% Senior Notes Due 2021 [Member] [Member]
Debt Instrument [Line Items]
Interest rate on senior notes						7.375%
Unsecured Debt [Member] | 8.0% Senior Notes Due 2020 [Domain]
Debt Instrument [Line Items]
Interest rate on senior notes						8.00%
Unsecured Debt [Member] | Recourse Debt [Member] | 4.0% Senior Notes Due 2021 [Domain] [Domain]
Debt Instrument [Line Items]
Gain (loss) on extinguishment of debt				$ (125)
Unsecured Debt [Member] | Recourse Debt [Member] | 5.5% Senior Notes Due 2024 [Member] [Member]
Debt Instrument [Line Items]
Gain (loss) on extinguishment of debt				(44)
Unsecured Debt [Member] | Recourse Debt [Member] | 8.0% Senior Notes Due 2020 [Domain]
Debt Instrument [Line Items]
Gain (loss) on extinguishment of debt			(47)	$ (36)
Secured Debt [Member] | Recourse Debt [Member] | LIBOR 2.00% Senior Notes Due in 2022 [Member]
Debt Instrument [Line Items]
Gain (loss) on extinguishment of debt			$ (6)
London Interbank Offered Rate (LIBOR) [Member] | LIBOR 2.00% Senior Notes Due in 2022 [Member] | Recourse Debt [Member]
Debt Instrument [Line Items]
Debt Instrument, Basis Spread on Variable Rate				2.00%
London Interbank Offered Rate (LIBOR) [Member] | Senior Unsecured Note LIBOR plus 3% due 2019 [Member] | Recourse Debt [Member]
Debt Instrument [Line Items]
Debt Instrument, Basis Spread on Variable Rate					3.00%