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Debt and Capital Leases - Recourse Debt Issuances/Repurchases (Details) - USD ($) $ in Millions				12 Months Ended
	Dec. 07, 2017	Aug. 02, 2016	May 23, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Debt Instrument
Loss on debt extinguishment				$ 53	$ 142	$ (10)
Senior notes
Debt Instrument
Accrued interest					77
Loss on debt extinguishment					117
Deferred financing costs					$ 16
Recourse Debt | Senior Notes due 2028
Debt Instrument
Proceeds from issuance of senior notes	$ 870
Interest rate, stated percentage	5.75%			5.75%
Recourse Debt | Senior Notes due 2026
Debt Instrument
Proceeds from issuance of senior notes			$ 1,000
Interest rate, stated percentage			7.25%	7.25%
Recourse Debt | Senior Notes due 2027
Debt Instrument
Proceeds from issuance of senior notes		$ 1,250
Interest rate, stated percentage		6.625%		6.625%
Recourse Debt | Senior notes, due 2020
Debt Instrument
Interest rate, stated percentage		8.25%			8.25%
Principal Repurchased					$ 1,058
Cash Paid					$ 1,129
Average Early Redemption Percentage					103.12%
Recourse Debt | Senior notes, due 2018
Debt Instrument
Interest rate, stated percentage				7.625%	7.625%
Principal Repurchased				$ 398	$ 641
Cash Paid				$ 411	$ 706
Average Early Redemption Percentage				101.42%	107.89%
Redemptions by cash				$ 186
Recourse Debt | Senior notes, due 2021
Debt Instrument
Interest rate, stated percentage		7.875%		7.875%	7.875%
Principal Repurchased				$ 206	$ 922
Cash Paid				$ 218	$ 978
Average Early Redemption Percentage				102.63%	104.00%
Redemptions by cash				$ 193
Recourse Debt | Senior notes, due 2023
Debt Instrument
Interest rate, stated percentage	6.625%			6.625%	6.625%
Principal Repurchased				$ 869	$ 67
Cash Paid				$ 915	$ 64
Average Early Redemption Percentage				103.57%	94.13%
Recourse Debt | Senior notes, due 2022
Debt Instrument
Interest rate, stated percentage				6.25%	6.25%
Principal Repurchased					$ 108
Cash Paid					$ 105
Average Early Redemption Percentage				35.00%	94.73%
Recourse Debt | Senior notes
Debt Instrument
Principal amount redeemed				$ 1,500
Principal Repurchased				1,473	$ 2,967
Cash Paid				$ 1,544	$ 3,145
Average Early Redemption Percentage				25.00%
Accrued interest				$ 29
Loss on debt extinguishment				49
Write off of deferred debt issuance cost				$ 7
Recourse Debt | Senior Notes 2024
Debt Instrument
Interest rate, stated percentage				6.25%	6.25%
Principal Repurchased					$ 171
Cash Paid					$ 163
Average Early Redemption Percentage				35.00%	94.52%

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-Publisher FASB
-Name Accounting Standards Codification
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-SubTopic 10
-Section S99
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