Debt - Schedule of Debt (Details) - USD ($) $ in Millions	Sep. 30, 2018	Dec. 31, 2017
Short Term and Long Term Debt [Line Items]
Secured debt	$ 39,722	$ 39,887
Unsecured debt	46,655	40,830
Total secured and unsecured debt	86,377	80,717
Secured debt fair value	39,679	39,948
Unsecured debt fair value	47,062	41,989
Fair value of Secured and Unsecured debt	86,741	81,937
Level 2 [Member]
Short Term and Long Term Debt [Line Items]
Fair value of Secured and Unsecured debt	84,693	79,623
Level 3 [Member]
Short Term and Long Term Debt [Line Items]
Fair value of Secured and Unsecured debt	2,048	2,314
Revolving Credit Facility [Member]
Short Term and Long Term Debt [Line Items]
Secured debt	2,327	4,694
Unsecured debt	2,118	2,368
Secured debt fair value	2,331	4,713
Unsecured debt fair value	2,114	2,375
Securitization Notes Payable [Member]
Short Term and Long Term Debt [Line Items]
Secured debt	37,395	35,193
Secured debt fair value	37,348	35,235
Senior Notes [Member]
Short Term and Long Term Debt [Line Items]
Unsecured debt	41,004	36,820
Unsecured debt fair value	41,417	37,969
Other Unsecured Debt [Member]
Short Term and Long Term Debt [Line Items]
Unsecured debt	3,533	1,642
Unsecured debt fair value	$ 3,531	$ 1,645