General (Impact of Adoption on Condensed Balance Sheet and Condensed Consolidated Statements of Operations) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Jan. 01, 2018	Dec. 31, 2017
Statement of operations:
Non-insurance warranty revenue	$ 258	$ 99	$ 744	$ 290
Total revenues	2,622	2,398	7,731	7,094
Non-insurance warranty expense	235	74	676	216
Total claims, benefits and expenses	2,220	2,211	6,653	6,194
Income before income tax	402	187	1,078	900
Income tax expense	(66)	(43)	(181)	(224)
Net income	336	$ 144	897	$ 676
Balance sheets:
Other assets	3,589		3,589		$ 3,015	$ 1,133
Other liabilities	6,571		6,571		6,222	4,253
Deferred income taxes	314		314		158	137
Retained earnings	9,456		9,456		9,348	9,414
Prior to Adoption
Statement of operations:
Non-insurance warranty revenue	109		315
Total revenues	2,473		7,302
Non-insurance warranty expense	82		240
Total claims, benefits and expenses	2,067		6,217
Income before income tax	406		1,085
Income tax expense	(67)		(183)
Net income	339		902
Balance sheets:
Other assets	3,284		3,284			1,133
Other liabilities	6,259		6,259			4,253
Deferred income taxes	312		312			137
Retained earnings	9,461		9,461			$ 9,414
Effect of Adoption
Statement of operations:
Non-insurance warranty revenue	149		429
Total revenues	149		429
Non-insurance warranty expense	153		436
Total claims, benefits and expenses	153		436
Income before income tax	(4)		(7)
Income tax expense	1		2
Net income	(3)		(5)
Balance sheets:
Other assets	305		305		1,882
Other liabilities	312		312		1,969
Deferred income taxes	2		2		21
Retained earnings	$ (5)		$ (5)		$ (66)