Repurchase Agreements (Details) - USD ($) $ in Thousands	Mar. 31, 2016	Dec. 31, 2015
Assets Sold under Agreements to Repurchase [Line Items]
Securities Sold under Agreements to Repurchase, Fair Value of Collateral	$ 787,000	$ 722,000
Securities Sold under Agreements to Repurchase	716,685	622,182
Securities Sold under Agreements to Repurchase, Gross	716,685	622,182
Government Agency
Assets Sold under Agreements to Repurchase [Line Items]
Securities Sold under Agreements to Repurchase	4,280	4,276
U. S. Treasury
Assets Sold under Agreements to Repurchase [Line Items]
Securities Sold under Agreements to Repurchase	712,405	617,906
Overnight and continuous
Assets Sold under Agreements to Repurchase [Line Items]
Securities Sold under Agreements to Repurchase	686,685	592,182
Overnight and continuous | Government Agency
Assets Sold under Agreements to Repurchase [Line Items]
Securities Sold under Agreements to Repurchase	0	0
Overnight and continuous | U. S. Treasury
Assets Sold under Agreements to Repurchase [Line Items]
Securities Sold under Agreements to Repurchase	686,685	592,182
Up to 30 days
Assets Sold under Agreements to Repurchase [Line Items]
Securities Sold under Agreements to Repurchase	0	0
Up to 30 days | Government Agency
Assets Sold under Agreements to Repurchase [Line Items]
Securities Sold under Agreements to Repurchase	0	0
Up to 30 days | U. S. Treasury
Assets Sold under Agreements to Repurchase [Line Items]
Securities Sold under Agreements to Repurchase	0	0
30 - 90 days
Assets Sold under Agreements to Repurchase [Line Items]
Securities Sold under Agreements to Repurchase	0	0
30 - 90 days | Government Agency
Assets Sold under Agreements to Repurchase [Line Items]
Securities Sold under Agreements to Repurchase	0	0
30 - 90 days | U. S. Treasury
Assets Sold under Agreements to Repurchase [Line Items]
Securities Sold under Agreements to Repurchase	0	0
Greater than 90 days
Assets Sold under Agreements to Repurchase [Line Items]
Securities Sold under Agreements to Repurchase	30,000	30,000
Greater than 90 days | Government Agency
Assets Sold under Agreements to Repurchase [Line Items]
Securities Sold under Agreements to Repurchase	4,280	4,276
Greater than 90 days | U. S. Treasury
Assets Sold under Agreements to Repurchase [Line Items]
Securities Sold under Agreements to Repurchase	$ 25,720	$ 25,724