Repurchase Agreement Borrowings (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2017	Dec. 31, 2016	Nov. 30, 2012
Assets and Associated Liabilities of Transfers Accounted for as Secured Borrowings [Line Items]
Transfers Accounted for as Secured Borrowings, Associated Liabilities, Carrying Amount	$ 519,863	$ 509,795
Pledged Assets Separately Reported, Securities Pledged for Repurchase Agreements, at Fair Value	594,000	569,000
Available unpledged securities	570,000	640,000
Long-term Debt [Member]
Assets and Associated Liabilities of Transfers Accounted for as Secured Borrowings [Line Items]
Long-term Debt, Prepayment Penalty	25,000
Transfers Accounted for as Secured Borrowings, Associated Liabilities, Carrying Amount			$ 300,000
Payments of Debt Extinguishment Costs, Unamortized Balance	3,500	4,700
Maturity Overnight [Member]
Assets and Associated Liabilities of Transfers Accounted for as Secured Borrowings [Line Items]
Transfers Accounted for as Secured Borrowings, Associated Liabilities, Carrying Amount	218,759	208,691
Maturity up to 30 days [Member]
Assets and Associated Liabilities of Transfers Accounted for as Secured Borrowings [Line Items]
Transfers Accounted for as Secured Borrowings, Associated Liabilities, Carrying Amount	0	0
Maturity 30 to 90 Days [Member]
Assets and Associated Liabilities of Transfers Accounted for as Secured Borrowings [Line Items]
Transfers Accounted for as Secured Borrowings, Associated Liabilities, Carrying Amount	0	0
Maturity over 90 days [Member]
Assets and Associated Liabilities of Transfers Accounted for as Secured Borrowings [Line Items]
Transfers Accounted for as Secured Borrowings, Associated Liabilities, Carrying Amount	$ 301,104	$ 301,104