Repurchase Agreements (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Assets Sold under Agreements to Repurchase [Line Items]
Securities Sold under Agreements to Repurchase, Fair Value of Collateral	$ 511,800	$ 684,200
U. S. Treasury
Assets Sold under Agreements to Repurchase [Line Items]
Securities Sold under Agreements to Repurchase		586,171
Overnight and continuous | U. S. Treasury
Assets Sold under Agreements to Repurchase [Line Items]
Securities Sold under Agreements to Repurchase	$ 499,723	556,171
30 - 90 days | U. S. Treasury
Assets Sold under Agreements to Repurchase [Line Items]
Securities Sold under Agreements to Repurchase		$ 30,000