Carrying Value of Repurchase Agreements (Details) - USD ($) $ in Thousands	Jun. 30, 2017	Dec. 31, 2016
Securities Sold Under Agreements to Repurchase
Repurchase Agreements	$ 451,050	$ 473,650
Overnight and Continuous
Securities Sold Under Agreements to Repurchase
Repurchase Agreements	451,050	473,007
Up to 30 Days
Securities Sold Under Agreements to Repurchase
Repurchase Agreements	0	643
Residential mortgage-backed securities
Securities Sold Under Agreements to Repurchase
Repurchase Agreements	440,955	472,349
Residential mortgage-backed securities | Overnight and Continuous
Securities Sold Under Agreements to Repurchase
Repurchase Agreements	440,955	471,706
Residential mortgage-backed securities | Up to 30 Days
Securities Sold Under Agreements to Repurchase
Repurchase Agreements	0	643
Commercial mortgage-backed securities
Securities Sold Under Agreements to Repurchase
Repurchase Agreements	10,095	1,301
Commercial mortgage-backed securities | Overnight and Continuous
Securities Sold Under Agreements to Repurchase
Repurchase Agreements	10,095	1,301
Commercial mortgage-backed securities | Up to 30 Days
Securities Sold Under Agreements to Repurchase
Repurchase Agreements	$ 0	$ 0