Summary of Significant Accounting Policies (Cumulative Effect to the Consolidated Balance Sheet) (Details) - USD ($) $ in Millions	Dec. 31, 2018	Jan. 01, 2018	Dec. 31, 2017
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Deferred non-insurance warranty acquisition expense	$ 2,513	$ 2,094	$ 212
Deferred non-insurance warranty revenue	3,402	2,941	972
Deferred income taxes	392	158	137
Retained earnings	9,277	9,348	9,414
Balance as of December 31, 2017
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Deferred non-insurance warranty acquisition expense	2,116		212
Deferred non-insurance warranty revenue	2,997		972
Deferred income taxes	390		137
Retained earnings	9,283		$ 9,414
Adjustments Due to Adoption of Topic 606
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Deferred non-insurance warranty acquisition expense	397	1,882
Deferred non-insurance warranty revenue	405	1,969
Deferred income taxes	2	21
Retained earnings	$ (6)	$ (66)