Summary of Significant Accounting Policies (Impact of adoption on the Consolidated Statements of Operations and Balance Sheet) (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Jan. 01, 2018
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Non-insurance warranty revenue									$ 1,007	$ 390	$ 361
Revenues	$ 2,403	$ 2,622	$ 2,574	$ 2,535	$ 2,448	$ 2,398	$ 2,366	$ 2,330	10,134	9,542	9,366
Non-insurance warranty expense									923	299	271
Total claims, benefits and expenses									9,170	8,232	8,229
Income before income tax									964	1,310	1,137
Income tax expense									(151)	(411)	(278)
Net income	(84)	$ 336	$ 270	$ 291	223	$ 144	$ 272	$ 260	813	899	$ 859
Deferred non-insurance warranty acquisition expense	2,513				212				2,513	212		$ 2,094
Deferred non-insurance warranty revenue	3,402				972				3,402	972		2,941
Deferred income taxes	392				137				392	137		158
Retained earnings	9,277				9,414				9,277	9,414		9,348
Prior to Adoption
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Non-insurance warranty revenue									420
Revenues									9,547
Non-insurance warranty expense									328
Total claims, benefits and expenses									8,575
Income before income tax									972
Income tax expense									(153)
Net income									819
Deferred non-insurance warranty acquisition expense	2,116				212				2,116	212
Deferred non-insurance warranty revenue	2,997				972				2,997	972
Deferred income taxes	390				137				390	137
Retained earnings	9,283				$ 9,414				9,283	$ 9,414
Effect of Adoption
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Non-insurance warranty revenue									587
Revenues									587
Non-insurance warranty expense									595
Total claims, benefits and expenses									595
Income before income tax									(8)
Income tax expense									2
Net income									(6)
Deferred non-insurance warranty acquisition expense	397								397			1,882
Deferred non-insurance warranty revenue	405								405			1,969
Deferred income taxes	2								2			21
Retained earnings	$ (6)								$ (6)			$ (66)