Borrowings - Short Term Borrowings (Details) - USD ($)	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015
Short-term borrowings:
Short-Term	$ 0	$ 500,175,000
Maximum outstanding at any month end	526,500,000	710,005,000
ABCP Facility
Short-term borrowings:
Short-Term	$ 0	$ 500,175,000
Weighted Average Interest Rate	0.00%	0.84%
Average Balance	$ 229,719,000	$ 135,064,000
Weighted Average Interest Rate	2.61%	3.10%