Balance Sheet Offsetting (Assets and liabilities subject to MNA, or repurchase agreements) (Details 2) - USD ($) $ in Thousands	Sep. 30, 2015	Dec. 31, 2014
Offsetting Assets and Liabilities [Line items]
Deposits, Government Entities	$ 12,936,962	$ 12,633,089
Liabilities:
Repurchase Agreements, Gross Amounts of Recognized Liabilities	632,138	688,601
Repurchase Agreements, Gross Amounts Offset in the Statements of Condition	0	0
Repurchase Agreements, Net Amounts of Liabilities Presented in the Statements of Condition	632,138	688,601
Repurchase Agreements, Fair Value of Collateral Pledged	632,138	688,601
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	15,600	16,200
Institutional Counterparties | Interest Rate Swap Agreements
Assets:
Gross Amounts of Recognized Assets	305	28
Gross Amounts Offset in the Statements of Condition	0	0
Net Amounts of Assets Presented in the Statements of Condition	305	28
Netting Adjustments per Master Netting Arrangements	305	28
Gross Amounts Not Offset in the Statements of Condition - FV of collateral pledged	0	0
Derivative Assets, Net Amount	0	0
Liabilities:
Gross Amounts of Recognized Liabilities	15,907	16,268
Gross Amounts Offset in the Statements of Condition	0	0
Net Amounts of Liabilities Presented in the Statements of Condition	15,907	16,268
Derivative Liability, Netting Adjustments per Master Netting Arrangements	305	28
Derivative, Collateral, Right to Reclaim Securities	0	0
Derivative Liabilities, Net Amount	15,602	16,240
Private Institutions
Offsetting Assets and Liabilities [Line items]
Securities Sold under Agreements to Repurchase, Fair Value of Collateral	700,000	700,000
Liabilities:
Repurchase Agreements, Gross Amounts of Recognized Liabilities	575,000	600,000
Repurchase Agreements, Gross Amounts Offset in the Statements of Condition	0	0
Repurchase Agreements, Net Amounts of Liabilities Presented in the Statements of Condition	575,000	600,000
Repurchase Agreements, Fair Value of Collateral Pledged	575,000	600,000
Securities Sold under Agreements to Repurchase, Net Amount Offset Against Collateral	0	0
Government Entities
Offsetting Assets and Liabilities [Line items]
Deposits, Government Entities	1,200,000	1,300,000
Fair Value of Investment Securities Pledged, Government Entities	1,900,000	2,100,000
Liabilities:
Repurchase Agreements, Gross Amounts of Recognized Liabilities	57,138	88,601
Repurchase Agreements, Gross Amounts Offset in the Statements of Condition	0	0
Repurchase Agreements, Net Amounts of Liabilities Presented in the Statements of Condition	57,138	88,601
Repurchase Agreements, Fair Value of Collateral Pledged	57,138	88,601
Securities Sold under Agreements to Repurchase, Net Amount Offset Against Collateral	$ 0	$ 0