Debt - Short Term and Long Term Debt (Details) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017
Short Term and Long Term Debt [Line Items]
Secured debt	$ 42,835	$ 39,887
Secured Debt, Fair Value Disclosure	42,835	39,948
Unsecured debt	48,153	40,830
Unsecured Debt, Fair Value Disclosure	47,556	41,989
Long-term Debt	90,988	80,717
Long-term Debt, Fair Value	90,391	81,937
Fair Value, Inputs, Level 2 [Member]
Short Term and Long Term Debt [Line Items]
Long-term Debt, Fair Value	88,305	79,623
Fair Value, Inputs, Level 3 [Member]
Short Term and Long Term Debt [Line Items]
Long-term Debt, Fair Value	2,086	2,314
Revolving Credit Facility [Member]
Short Term and Long Term Debt [Line Items]
Secured debt	3,410	4,694
Secured Debt, Fair Value Disclosure	3,413	4,713
Unsecured debt	2,157	2,368
Unsecured Debt, Fair Value Disclosure	2,151	2,375
Securitization Notes Payable [Member]
Short Term and Long Term Debt [Line Items]
Secured debt	39,425	35,193
Secured Debt, Fair Value Disclosure	39,422	35,235
Senior Notes [Member]
Short Term and Long Term Debt [Line Items]
Unsecured debt	42,611	36,820
Unsecured Debt, Fair Value Disclosure	42,015	37,969
Other Unsecured Debt [Member]
Short Term and Long Term Debt [Line Items]
Unsecured debt	3,385	1,642
Unsecured Debt, Fair Value Disclosure	$ 3,390	$ 1,645