Segment Information - Schedule of Segment Reporting Information by Segment (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2018	Mar. 31, 2017	Mar. 31, 2016
Segment Reporting Information [Line Items]
Total financing revenues	$ 2,766	$ 2,706	$ 2,647	$ 2,598	$ 2,555	$ 2,537	$ 2,496	$ 2,458	$ 10,717	$ 10,046	$ 9,403
Depreciation on operating leases	1,863	1,778	1,719	1,681	1,859	1,722	1,683	1,589	7,041	6,853	5,914
Interest expense	523	428	452	448	449	701	297	307	1,851	1,754	1,137
Net financing revenues	380	500	476	469	247	114	516	562	1,825	1,439	2,352
Insurance earned premiums and contract revenues	225	220	221	216	210	202	199	193	882	804	719
Gain on sale of commercial finance business									0	0	197
Investment and other income, net	51	63	55	47	37	52	42	39	216	170	158
Realized gains (losses), net on investments in marketable securities	(4)	3	1	41	(14)	157	70	13	41	226	6
Net financing revenues and other revenues	652	786	753	773	480	525	827	807	2,964	2,639	3,432
Expenses:
Provision for credit losses	81	108	127	85	186	183	161	52	401	582	441
Operating and administrative expenses	384	323	337	313	356	325	317	279	1,357	1,277	1,161
Insurance losses and loss adjustment expenses	109	100	102	114	99	92	91	89	425	371	318
Total expenses	574	531	566	512	641	600	569	420	2,183	2,230	1,920
Income before income taxes	78	255	187	261	(161)	(75)	258	387	781	409	1,512
(Benefit) provision for income taxes	26	(2,821)	70	96	(70)	(29)	95	146	(2,629)	142	580
Net income	52	$ 3,076	$ 117	$ 165	(91)	$ (46)	$ 163	$ 241	3,410	267	932
Total assets	120,546				119,635				120,546	119,635	114,592
Intercompany Eliminations [Member]
Segment Reporting Information [Line Items]
Total financing revenues									0	0	0
Depreciation on operating leases									0	0	0
Interest expense									(12)	(5)	0
Net financing revenues									12	5	0
Insurance earned premiums and contract revenues									0	0	0
Gain on sale of commercial finance business											0
Investment and other income, net									(12)	(5)	0
Realized gains (losses), net on investments in marketable securities									0	0	0
Net financing revenues and other revenues									0	0	0
Expenses:
Provision for credit losses									0	0	0
Operating and administrative expenses									0	0	0
Insurance losses and loss adjustment expenses									0	0	0
Total expenses									0	0	0
Income before income taxes									0	0	0
(Benefit) provision for income taxes									0	0	0
Net income									0	0	0
Total assets	(1,087)				(1,083)				(1,087)	(1,083)	(1,065)
Finance Operations [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Total financing revenues									10,717	10,046	9,403
Depreciation on operating leases									7,041	6,853	5,914
Interest expense									1,863	1,759	1,137
Net financing revenues									1,813	1,434	2,352
Insurance earned premiums and contract revenues									0	0	0
Gain on sale of commercial finance business											197
Investment and other income, net									140	96	59
Realized gains (losses), net on investments in marketable securities									0	241	40
Net financing revenues and other revenues									1,953	1,771	2,648
Expenses:
Provision for credit losses									401	582	441
Operating and administrative expenses									1,028	979	909
Insurance losses and loss adjustment expenses									0	0	0
Total expenses									1,429	1,561	1,350
Income before income taxes									524	210	1,298
(Benefit) provision for income taxes									(2,654)	67	501
Net income									3,178	143	797
Total assets	116,942				116,242				116,942	116,242	111,496
Insurance Operations [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Total financing revenues									0	0	0
Depreciation on operating leases									0	0	0
Interest expense									0	0	0
Net financing revenues									0	0	0
Insurance earned premiums and contract revenues									882	804	719
Gain on sale of commercial finance business											0
Investment and other income, net									88	79	99
Realized gains (losses), net on investments in marketable securities									41	(15)	(34)
Net financing revenues and other revenues									1,011	868	784
Expenses:
Provision for credit losses									0	0	0
Operating and administrative expenses									329	298	252
Insurance losses and loss adjustment expenses									425	371	318
Total expenses									754	669	570
Income before income taxes									257	199	214
(Benefit) provision for income taxes									25	75	79
Net income									232	124	135
Total assets	$ 4,691				$ 4,476				$ 4,691	$ 4,476	$ 4,161