Debt - Short Term and Long Term Debt (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Short Term and Long Term Debt [Line Items]
Secured debt	$ 39,887	$ 35,087
Secured Debt, Fair Value Disclosure	39,948	35,162
Unsecured debt	40,830	29,476
Unsecured Debt, Fair Value Disclosure	41,989	30,045
Long-term Debt	80,717	64,563
Long-term Debt, Fair Value	81,937	65,207
Fair Value, Inputs, Level 2 [Member]
Short Term and Long Term Debt [Line Items]
Long-term Debt, Fair Value	79,623	62,951
Fair Value, Inputs, Level 3 [Member]
Short Term and Long Term Debt [Line Items]
Long-term Debt, Fair Value	2,314	2,256
Revolving Credit Facility [Member]
Short Term and Long Term Debt [Line Items]
Secured debt	4,694	8,503
Secured Debt, Fair Value Disclosure	4,713	8,498
Unsecured debt	2,368	1,961
Unsecured Debt, Fair Value Disclosure	2,375	1,961
Securitization Notes Payable [Member]
Short Term and Long Term Debt [Line Items]
Secured debt	35,193	26,584
Secured Debt, Fair Value Disclosure	35,235	26,664
Senior Notes [Member]
Short Term and Long Term Debt [Line Items]
Unsecured debt	36,820	26,737
Unsecured Debt, Fair Value Disclosure	37,969	27,304
Other Unsecured Debt [Member]
Short Term and Long Term Debt [Line Items]
Unsecured debt	1,642	778
Unsecured Debt, Fair Value Disclosure	$ 1,645	$ 780