Debt - Narrative (Details) - USD ($) $ in Billions	3 Months Ended
	Mar. 31, 2018	Apr. 26, 2018	Dec. 31, 2017
Secured Debt [Member] | Securitization Notes Payable [Member]
Debt Instrument [Line Items]
Debt instrument, face amount	$ 4.7
Weighted average interest rate	2.59%
Senior Notes [Member]
Debt Instrument [Line Items]
Debt instrument, face amount	$ 40.8		$ 37.3
Increase in Senior Notes during the period	3.4
Senior Notes [Member] | Senior Notes Issued By Subsidiaries in Canada and Mexico [Member]
Debt Instrument [Line Items]
Debt instrument, face amount	$ 1.2
Parent Company [Member] | Senior Notes [Member] | Fixed Rate Senior Notes [Member]
Debt Instrument [Line Items]
Weighted average interest rate	2.30%
Subsequent Event [Member] | Parent Company [Member] | Senior Notes [Member]
Debt Instrument [Line Items]
Debt instrument, face amount		$ 2.5
Weighted average interest rate		3.80%