Derivative Instruments and Other Hedging Activities - Reconciliation of Gross Amounts in Consolidated Balance Sheet (Detail) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012
Derivative Instruments And Hedging Activities [Line Items]
Derivative assets subject to master netting arrangements, Gross Amounts	$ 34,504	$ 35,202
Derivative liabilities subject to master netting arrangements, Gross Amounts	22,649	27,783
Derivative assets subject to master netting arrangements, Net Amount	34,319	34,703
Derivative liabilities subject to master netting arrangements, Net Amount	10,147	13,934
Derivatives [Member]
Derivative Instruments And Hedging Activities [Line Items]
Derivative assets subject to master netting arrangements, Gross Amounts Not Offset	(185)	(499)
Derivative liabilities subject to master netting arrangements, Gross Amounts Not Offset	(185)	(499)
Collateral [Member]
Derivative Instruments And Hedging Activities [Line Items]
Derivative assets subject to master netting arrangements, Gross Amounts Not Offset
Derivative liabilities subject to master netting arrangements, Gross Amounts Not Offset	(12,317)	(13,350)
Interest Rate Contracts [Member] | Designated As Hedging Instruments [Member]
Derivative Instruments And Hedging Activities [Line Items]
Derivative assets subject to master netting arrangements, Gross Amounts	1,744	499
Derivative liabilities subject to master netting arrangements, Gross Amounts	590	1,843
Derivative assets subject to master netting arrangements, Net Amount	1,559
Derivative liabilities subject to master netting arrangements, Net Amount	405	1,344
Interest Rate Contracts [Member] | Not Designated As Hedging Instruments [Member]
Derivative Instruments And Hedging Activities [Line Items]
Derivative assets subject to master netting arrangements, Gross Amounts	22,058	25,940
Derivative liabilities subject to master netting arrangements, Gross Amounts	22,059	25,940
Derivative assets subject to master netting arrangements, Net Amount	22,058	25,940
Derivative liabilities subject to master netting arrangements, Net Amount	9,742	12,590
Interest Rate Contracts [Member] | Derivatives [Member] | Designated As Hedging Instruments [Member]
Derivative Instruments And Hedging Activities [Line Items]
Derivative assets subject to master netting arrangements, Gross Amounts Not Offset	(185)	(499)
Derivative liabilities subject to master netting arrangements, Gross Amounts Not Offset	(185)	(499)
Interest Rate Contracts [Member] | Derivatives [Member] | Not Designated As Hedging Instruments [Member]
Derivative Instruments And Hedging Activities [Line Items]
Derivative assets subject to master netting arrangements, Gross Amounts Not Offset
Derivative liabilities subject to master netting arrangements, Gross Amounts Not Offset
Interest Rate Contracts [Member] | Collateral [Member] | Designated As Hedging Instruments [Member]
Derivative Instruments And Hedging Activities [Line Items]
Derivative assets subject to master netting arrangements, Gross Amounts Not Offset
Derivative liabilities subject to master netting arrangements, Gross Amounts Not Offset
Interest Rate Contracts [Member] | Collateral [Member] | Not Designated As Hedging Instruments [Member]
Derivative Instruments And Hedging Activities [Line Items]
Derivative assets subject to master netting arrangements, Gross Amounts Not Offset
Derivative liabilities subject to master netting arrangements, Gross Amounts Not Offset	(12,317)	(13,350)
Written And Purchased Options [Member]
Derivative Instruments And Hedging Activities [Line Items]
Derivative assets subject to master netting arrangements, Gross Amounts	10,702	8,763
Derivative assets subject to master netting arrangements, Net Amount	10,702	8,763
Written And Purchased Options [Member] | Derivatives [Member]
Derivative Instruments And Hedging Activities [Line Items]
Derivative assets subject to master netting arrangements, Gross Amounts Not Offset
Written And Purchased Options [Member] | Collateral [Member]
Derivative Instruments And Hedging Activities [Line Items]
Derivative assets subject to master netting arrangements, Gross Amounts Not Offset