Repurchase Agreements (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Assets Sold under Agreements to Repurchase [Line Items]
Securities Sold under Agreements to Repurchase, Gross	$ 586,171	$ 620,772
Securities Sold under Agreements to Repurchase, Fair Value of Collateral	684,200	690,800
U. S. Treasury
Assets Sold under Agreements to Repurchase [Line Items]
Securities Sold under Agreements to Repurchase	586,171	620,772
U. S. Treasury | Overnight and Continuous
Assets Sold under Agreements to Repurchase [Line Items]
Securities Sold under Agreements to Repurchase	556,171	590,772
U. S. Treasury | Up to 30 Days
Assets Sold under Agreements to Repurchase [Line Items]
Securities Sold under Agreements to Repurchase	0	0
U. S. Treasury | 30 - 90 Days
Assets Sold under Agreements to Repurchase [Line Items]
Securities Sold under Agreements to Repurchase	30,000	0
U. S. Treasury | Greater than 90 Days
Assets Sold under Agreements to Repurchase [Line Items]
Securities Sold under Agreements to Repurchase	$ 0	$ 30,000