Debt - Schedule of Debt (Details) - USD ($) $ in Millions	Jun. 30, 2018	Dec. 31, 2017
Short Term and Long Term Debt [Line Items]
Secured debt	$ 39,083	$ 39,887
Unsecured debt	45,357	40,830
Total secured and unsecured debt	84,440	80,717
Secured debt fair value	39,024	39,948
Unsecured debt fair value	45,845	41,989
Fair value of Secured and Unsecured debt	84,869	81,937
Level 2 [Member]
Short Term and Long Term Debt [Line Items]
Fair value of Secured and Unsecured debt	82,604	79,623
Level 3 [Member]
Short Term and Long Term Debt [Line Items]
Fair value of Secured and Unsecured debt	2,265	2,314
Revolving Credit Facility [Member]
Short Term and Long Term Debt [Line Items]
Secured debt	2,945	4,694
Unsecured debt	2,095	2,368
Secured debt fair value	2,949	4,713
Unsecured debt fair value	2,095	2,375
Securitization Notes Payable [Member]
Short Term and Long Term Debt [Line Items]
Secured debt	36,138	35,193
Secured debt fair value	36,075	35,235
Senior Notes [Member]
Short Term and Long Term Debt [Line Items]
Unsecured debt	40,916	36,820
Unsecured debt fair value	41,400	37,969
Other Unsecured Debt [Member]
Short Term and Long Term Debt [Line Items]
Unsecured debt	2,346	1,642
Unsecured debt fair value	$ 2,350	$ 1,645