Securities Sold Under Agreements to Repurchase (Details) - USD ($) $ in Thousands	Mar. 31, 2017	Dec. 31, 2016
Year
Total	$ 1,100,000	$ 1,100,000
Secured Debt
Year
2018	200,000
2019	600,000
2020	300,000
Total	$ 1,100,000
Secured Debt | Residential mortgage-backed securities
Year
Composition of outstanding repo agreements (as a percent)	82.00%
Secured Debt | Obligations of GSEs
Year
Composition of outstanding repo agreements (as a percent)	18.00%