Repurchase Agreement Borrowings Repurchase Agreement Borrowings (Details) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 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	$ 492,896	$ 509,795
Pledged Assets Separately Reported, Securities Pledged for Repurchase Agreements, at Fair Value	540,000	569,000
Available unpledged securities	619,000	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	868	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	191,792	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	300,000	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	$ 1,104	$ 301,104