Borrowings - Long Term Borrowings (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Line of Credit Facility [Line Items]
Long-term borrowings	$ 4,284,304	$ 3,275,270
Weighted Average Interest Rate	3.27%	2.71%
Average Balance	$ 3,893,533	$ 2,843,593
Floating-rate borrowings
Line of Credit Facility [Line Items]
Long-term borrowings	$ 1,802,609	$ 1,512,970
Weighted Average Interest Rate	3.26%	2.32%
Average Balance	$ 1,720,540	$ 1,422,856
Fixed-rate borrowings
Line of Credit Facility [Line Items]
Long-term borrowings	$ 2,481,695	$ 1,762,300
Weighted Average Interest Rate	3.28%	3.04%
Average Balance	$ 2,172,993	$ 1,420,737