Borrowings - Short-Term Credit Facilities (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Short-Term Credit Facilities
Borrowing Outstanding	$ 1,431,059
Interest expense	$ 64,107	$ 28,327	$ 29,254
Short-Term Credit Facilities
Short-Term Credit Facilities
Weighted average interest rate	3.86%	3.12%
Financing Available	$ 543,000	$ 493,000
Borrowing Outstanding	205,677	309,086
Interest expense	6,600	$ 6,300	$ 5,500
Revolving credit facility
Short-Term Credit Facilities
Financing Available	500,000
Borrowing Outstanding	7,000
Interest expense	$ 19