Repurchase Agreements (Details) - USD ($) $ in Thousands	Jun. 30, 2015	Dec. 31, 2014
Assets Sold under Agreements to Repurchase [Line Items]
Securities Sold under Agreements to Repurchase, Fair Value of Collateral	$ 869,000	$ 418,300
Securities Sold under Agreements to Repurchase	740,674	324,425
Securities Sold under Agreements to Repurchase, Gross	740,674	324,425
Government Agency
Assets Sold under Agreements to Repurchase [Line Items]
Securities Sold under Agreements to Repurchase	6,908	6,914
U. S. Treasury
Assets Sold under Agreements to Repurchase [Line Items]
Securities Sold under Agreements to Repurchase	733,766	186,010
Mortgage Backed Securities, Other [Member]
Assets Sold under Agreements to Repurchase [Line Items]
Securities Sold under Agreements to Repurchase		131,501
Overnight and continuous
Assets Sold under Agreements to Repurchase [Line Items]
Securities Sold under Agreements to Repurchase	710,674	294,425
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	710,674	162,924
Overnight and continuous | Mortgage Backed Securities, Other [Member]
Assets Sold under Agreements to Repurchase [Line Items]
Securities Sold under Agreements to Repurchase		131,501
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
Up to 30 days | Mortgage Backed Securities, Other [Member]
Assets Sold under Agreements to Repurchase [Line Items]
Securities Sold under Agreements to Repurchase		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
30 - 90 days | Mortgage Backed Securities, Other [Member]
Assets Sold under Agreements to Repurchase [Line Items]
Securities Sold under Agreements to Repurchase		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	6,908	6,914
Greater than 90 days | U. S. Treasury
Assets Sold under Agreements to Repurchase [Line Items]
Securities Sold under Agreements to Repurchase	$ 23,092	23,086
Greater than 90 days | Mortgage Backed Securities, Other [Member]
Assets Sold under Agreements to Repurchase [Line Items]
Securities Sold under Agreements to Repurchase		$ 0