Balance Sheet Offsetting Balance Sheet Offsetting - Remaining Contractual Maturity of the Securities Sold Under Agreement to Repurchase (Details) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
	Mar. 31, 2016	Dec. 31, 2015
Remaining Contractual Maturity of the Securities Sold Under Agreement to Repurchase [Line Items]
Securities Sold under Agreements to Repurchase, Gross	$ 685,036	$ 883,947
Securities Sold under Agreements to Repurchase, Collateral, Right to Reclaim Securities	0	0
Remaining contractual maturity of the agreements overnight and continuous	682,034	875,920
Remaining contractual maturity of the agreements up to 30 Days	0	0
Remaining contractual maturity of the agreements 30 to 90 Days	3,002	8,027
Remaining contractual maturity of the agreements greater than 90 Days	0	0
Remaining contractual maturity of the agreements total	685,036	883,947
Securities Sold under Agreements to Repurchase, Not Subject to Master Netting Arrangement	685,036	883,947
US Treasury Securities [Member]
Remaining Contractual Maturity of the Securities Sold Under Agreement to Repurchase [Line Items]
Remaining contractual maturity of the agreements overnight and continuous	515,533	664,419
Remaining contractual maturity of the agreements up to 30 Days	0	0
Remaining contractual maturity of the agreements 30 to 90 Days	0	0
Remaining contractual maturity of the agreements greater than 90 Days	0	0
Remaining contractual maturity of the agreements total	515,533	664,419
Residential mortgage-backed securities [Member]
Remaining Contractual Maturity of the Securities Sold Under Agreement to Repurchase [Line Items]
Remaining contractual maturity of the agreements overnight and continuous	166,501	211,501
Remaining contractual maturity of the agreements up to 30 Days	0	0
Remaining contractual maturity of the agreements 30 to 90 Days	3,002	8,027
Remaining contractual maturity of the agreements greater than 90 Days	0	0
Remaining contractual maturity of the agreements total	$ 169,503	$ 219,528