Carrying Value of Repurchase Agreements (Details) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Securities Sold Under Agreements to Repurchase
Repurchase Agreements	$ 453,596	$ 473,650
Overnight and Continuous
Securities Sold Under Agreements to Repurchase
Repurchase Agreements	453,596	473,007
Up to 30 Days
Securities Sold Under Agreements to Repurchase
Repurchase Agreements	0	643
U.S. government and federal agency
Securities Sold Under Agreements to Repurchase
Repurchase Agreements	4,261
U.S. government and federal agency | Overnight and Continuous
Securities Sold Under Agreements to Repurchase
Repurchase Agreements	4,261
U.S. government and federal agency | Up to 30 Days
Securities Sold Under Agreements to Repurchase
Repurchase Agreements	0
Residential mortgage-backed securities
Securities Sold Under Agreements to Repurchase
Repurchase Agreements	438,872	472,349
Residential mortgage-backed securities | Overnight and Continuous
Securities Sold Under Agreements to Repurchase
Repurchase Agreements	438,872	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	10,463	1,301
Commercial mortgage-backed securities | Overnight and Continuous
Securities Sold Under Agreements to Repurchase
Repurchase Agreements	10,463	1,301
Commercial mortgage-backed securities | Up to 30 Days
Securities Sold Under Agreements to Repurchase
Repurchase Agreements	$ 0	$ 0