Borrowings - Summary of Information Related to Reverse Repurchase Agreements (Details) - Reverse repurchase agreements - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Borrowings
Average balance during the year	$ 1,100,000	$ 1,100,000	$ 1,100,000
Maximum balance at any month end during the year	1,100,000	1,100,000	1,100,000
Balance outstanding at end of year	$ 1,100,000	$ 1,100,000	$ 1,100,000
Weighted average interest rate during the year	3.62%	3.82%	4.06%
Weighted average interest rate at end of year	3.62%	3.62%	3.87%