Balance Sheet Offsetting (Assets and liabilities subject to MNA, or repurchase agreements) (Details 2) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Liabilities:
Repurchase Agreements, Gross Amounts of Recognized Liabilities	$ 505,293	$ 505,293
Repurchase Agreements, Gross Amounts Offset in the Statements of Condition	0	0
Repurchase Agreements, Net Amounts of Liabilities Presented in the Statements of Condition	505,293	505,293
Repurchase Agreements, Fair Value of Collateral Pledged	505,293	505,293
Securities Sold under Agreements to Repurchase, Net Amount Offset Against Collateral	0	0
Interest Rate Swap Agreements
Offsetting Assets and Liabilities [Line items]
Net liability positions, aggregate fair value	700	3,200
Institutional Counterparties | Interest Rate Swap Agreements
Offsetting Assets and Liabilities [Line items]
Derivative Liability, Fair Value of Collateral	400	3,500
Assets:
Gross Amounts of Recognized Assets	10,440	5,453
Gross Amounts Offset in the Statements of Condition	0	0
Net Amounts of Assets Presented in the Statements of Condition	10,440	5,453
Netting Adjustments per Master Netting Arrangements	1,898	4,017
Gross Amounts Not Offset in the Statements of Condition - FV of collateral pledged	(770)	0
Derivative Assets, Net Amount	7,772	1,436
Liabilities:
Gross Amounts of Recognized Liabilities	1,898	4,017
Gross Amounts Offset in the Statements of Condition	0	0
Net Amounts of Liabilities Presented in the Statements of Condition	1,898	4,017
Derivative Liability, Netting Adjustments per Master Netting Arrangements	1,898	4,017
Derivative, Collateral, Right to Reclaim Securities	0	0
Derivative Liabilities, Net Amount	0	0
Private Institutions
Offsetting Assets and Liabilities [Line items]
Securities Sold under Agreements to Repurchase, Fair Value of Collateral	546,900	563,300
Liabilities:
Repurchase Agreements, Gross Amounts of Recognized Liabilities	500,000	500,000
Repurchase Agreements, Gross Amounts Offset in the Statements of Condition	0	0
Repurchase Agreements, Net Amounts of Liabilities Presented in the Statements of Condition	500,000	500,000
Repurchase Agreements, Fair Value of Collateral Pledged	500,000	500,000
Securities Sold under Agreements to Repurchase, Net Amount Offset Against Collateral	0	0
Government Entities
Offsetting Assets and Liabilities [Line items]
Securities Sold under Agreements to Repurchase, Fair Value of Collateral	6,900	6,900
Liabilities:
Repurchase Agreements, Gross Amounts of Recognized Liabilities	5,293	5,293
Repurchase Agreements, Gross Amounts Offset in the Statements of Condition	0	0
Repurchase Agreements, Net Amounts of Liabilities Presented in the Statements of Condition	5,293	5,293
Repurchase Agreements, Fair Value of Collateral Pledged	5,293	5,293
Securities Sold under Agreements to Repurchase, Net Amount Offset Against Collateral	$ 0	$ 0