General (Impact of Adoption on Condensed Balance Sheet and Condensed Consolidated Statements of Operations) (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Jan. 01, 2018	Dec. 31, 2017
Statement of operations:
Non-insurance warranty revenue	$ 248	$ 98	$ 486	$ 191
Total revenues	2,574	2,366	5,109	4,696
Non-insurance warranty expense	225	72	441	142
Total claims, benefits and expenses	2,244	1,996	4,433	3,983
Income before income tax	330	370	676	713
Income tax expense	(60)	(98)	(115)	(181)
Net income	270	$ 272	561	$ 532
Balance sheets:
Other assets	3,426		3,426		$ 3,015	$ 1,133
Other liabilities	6,667		6,667		6,222	4,253
Deferred income taxes	296		296		158	137
Retained earnings	9,216		9,216		9,348	9,414
Prior to Adoption
Statement of operations:
Non-insurance warranty revenue	105		206
Total revenues	2,431		4,829
Non-insurance warranty expense	80		158
Total claims, benefits and expenses	2,099		4,150
Income before income tax	332		679
Income tax expense	(61)		(116)
Net income	271		563
Balance sheets:
Other assets	3,228		3,228			1,133
Other liabilities	6,466		6,466			4,253
Deferred income taxes	295		295			137
Retained earnings	9,218		9,218			$ 9,414
Effect of Adoption
Statement of operations:
Non-insurance warranty revenue	143		280
Total revenues	143		280
Non-insurance warranty expense	145		283
Total claims, benefits and expenses	145		283
Income before income tax	(2)		(3)
Income tax expense	1		1
Net income	(1)		(2)
Balance sheets:
Other assets	198		198		1,882
Other liabilities	201		201		1,969
Deferred income taxes	1		1		21
Retained earnings	$ (2)		$ (2)		$ (66)