Borrowings - Contractual Maturities of Reverse Repurchase Agreements (Details) - Reverse repurchase agreements - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Borrowings
2018	$ 200,000
2019	600,000
2020	300,000
Total	1,100,000	$ 1,100,000	$ 1,100,000
Borrowings callable in 2016	200,000
Borrowings callable in 2017	$ 100,000