Borrowings - Long Term Borrowings (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Line of Credit Facility [Line Items]
Long-term borrowings	$ 3,275,270	$ 2,167,979
Weighted Average Interest Rate	2.71%	2.15%
Average Balance	$ 2,843,593	$ 1,236,045
Floating rate borrowings
Line of Credit Facility [Line Items]
Long-term borrowings	$ 1,512,970	$ 1,175,819
Weighted Average Interest Rate	2.32%	1.71%
Average Balance	$ 1,422,856	$ 687,580
Fixed rate borrowings
Line of Credit Facility [Line Items]
Long-term borrowings	$ 1,762,300	$ 992,160
Weighted Average Interest Rate	3.04%	2.68%
Average Balance	$ 1,420,737	$ 548,465