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, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Jan. 01, 2018
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Non-insurance warranty revenue										$ 1,007
Revenues	$ 2,777	$ 2,685	$ 2,610	$ 2,695	$ 2,403	$ 2,622	$ 2,574	$ 2,535	$ 10,767	10,134	$ 9,542
Non-insurance warranty expense									1,082	923	299
Total claims, benefits and expenses									9,544	9,170	8,232
Income before income tax									1,223	964	1,310
Income tax expense									(223)	(151)	(411)
Net income	273	$ 107	$ 278	$ 342	(84)	$ 336	$ 270	$ 291	1,000	813	899
Deferred non-insurance warranty acquisition expense	2,840				2,513				2,840	2,513		$ 2,094
Deferred non-insurance warranty revenue	3,779				3,402				3,779	3,402		2,941
Deferred income taxes	199				392				199	392		158
Retained earnings	$ 9,348				9,277				$ 9,348	9,277		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					2,116	212
Deferred non-insurance warranty revenue					2,997					2,997	972
Deferred income taxes					390					390	137
Retained earnings					9,283					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)