Debt (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Short-term Debt [Line Items]
Outstanding Balance	$ 3,100,000	$ 3,278,000
Face amount of eligible notes issued	$ 2,000,000	$ 2,000,000
Short-term Debt [Member]
Short-term Debt [Line Items]
Year-End Stated Rate on Debt	2.13%	2.05%
Fees to maintain the secured financing facility	$ 7,800	$ 9,200
Commercial Paper [Member]
Short-term Debt [Line Items]
Outstanding Balance	$ 752,000	$ 1,168,000
Commercial Paper [Member] | Short-term Debt [Member]
Short-term Debt [Line Items]
Year-End Stated Rate on Debt	2.71%	1.54%
Other Short Term Borrowings [Member]
Short-term Debt [Line Items]
Outstanding Balance	$ 2,348,000	$ 2,110,000
Other Short Term Borrowings [Member] | Short-term Debt [Member]
Short-term Debt [Line Items]
Year-End Stated Rate on Debt	1.94%	2.34%