Balance Sheet Offsetting (Details) (USD $)	Sep. 30, 2013	Dec. 31, 2012
Liabilities:
Repurchase Agreements, Gross Amounts of Recognized Liabilities	$ 847,239,000	$ 758,947,000
Repurchase Agreements, Gross Amounts Offset in the Statements of Condition	0	0
Repurchase Agreements, Net Amounts of Liabilities Presented in the Statements of Condition	847,239,000	758,947,000
Repurchase Agreements,Collateral, Right to Reclaim Securities	847,239,000	758,947,000
Securities Sold under Agreements to Repurchase, Amount Offset Against Collateral	0	0
Government entity deposits	1,200,000,000	1,400,000,000
Interest Rate Swap Agreements
Offsetting Assets and Liabilities [Line items]
Net liability positions, aggregate fair value	23,200,000	32,400,000
Institutional Counterparties | Interest Rate Swap Agreements
Assets:
Gross Amounts of Recognized Assets	71,000
Gross Amounts Offset in the Statements of Condition	0
Net Amounts of Assets Presented in the Statements of Condition	71,000
Gross Amounts Not Offset in the Statements of Condition - Netting adj per MNA	71,000
Gross Amounts Not Offset in the Statements of Condition - FV of collateral pledged	0
Derivative Assets, Net Amount	0
Liabilities:
Gross Amounts of Recognized Liabilities	23,277,000	32,441,000
Gross Amounts Offset in the Statements of Condition	0	0
Net Amounts of Liabilities Presented in the Statements of Condition	23,277,000	32,441,000
Derivative Liability, Fair Value, Amount Not Offset Against Collateral	71,000	0
Derivative, Collateral, Right to Reclaim Securities	2,480,000	3,299,000
Derivative Liabilities, Net Amount	20,726,000	29,142,000
Private Institutions
Liabilities:
Repurchase Agreements, Gross Amounts of Recognized Liabilities	600,000,000	600,000,000
Repurchase Agreements, Gross Amounts Offset in the Statements of Condition	0	0
Repurchase Agreements, Net Amounts of Liabilities Presented in the Statements of Condition	600,000,000	600,000,000
Repurchase Agreements,Collateral, Right to Reclaim Securities	600,000,000	600,000,000
Securities Sold under Agreements to Repurchase, Amount Offset Against Collateral	0	0
Fair value of securities pledged	700,000,000	800,000,000
Government Entities
Liabilities:
Repurchase Agreements, Gross Amounts of Recognized Liabilities	247,239,000	158,947,000
Repurchase Agreements, Gross Amounts Offset in the Statements of Condition	0	0
Repurchase Agreements, Net Amounts of Liabilities Presented in the Statements of Condition	247,239,000	158,947,000
Repurchase Agreements,Collateral, Right to Reclaim Securities	247,239,000	158,947,000
Securities Sold under Agreements to Repurchase, Amount Offset Against Collateral	0	0
Fair value of investment securities pledged, not separately reported	$ 1,900,000,000	$ 2,200,000,000