Derivative Financial Instruments (Balance Sheet Netting) (Detail) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012
Derivative [Line Items]
Asset Derivatives	$ 24,690	$ 25,915
Liability Derivatives	67,111	72,126
Offsetting Of Financial Assets [Member] | Gross Amounts Of Recognized Assets [Member]
Derivative [Line Items]
Asset Derivatives	24,695	25,922
Offsetting Of Financial Assets [Member] | Gross Amounts Offset In Consolidated Balance Sheet [Member]
Derivative [Line Items]
Asset Derivatives	(5)	(7)
Offsetting Of Financial Assets [Member] | Net Amounts Of Assets Presented In Consolidated Balance Sheet [Member]
Derivative [Line Items]
Asset Derivatives	24,690	25,915
Offsetting Of Financial Assets [Member] | Financial Instruments Gross Amounts Not Offset In Consolidated Balance Sheet [Member]
Derivative [Line Items]
Asset Derivatives	0	0
Offsetting Of Financial Assets [Member] | Cash Collateral Received Gross Amounts Not Offset In Consolidated Balance Sheet [Member]
Derivative [Line Items]
Asset Derivatives	0	0
Offsetting Of Financial Assets [Member] | Net Amount [Member]
Derivative [Line Items]
Asset Derivatives	24,690	25,915
Offsetting Of Financial Liabilities [Member] | Gross Amounts Of Recognized Liabilities [Member]
Derivative [Line Items]
Liability Derivatives	67,111	72,126
Repurchase Agreements	305,251	302,577
Total Offsetting Financial Liabilities	372,362	374,703
Offsetting Of Financial Liabilities [Member] | Gross Amounts Offset In Consolidated Balance Sheet [Member]
Derivative [Line Items]
Liability Derivatives	0	0
Repurchase Agreements	0	0
Total Offsetting Financial Liabilities	0	0
Offsetting Of Financial Liabilities [Member] | Net Amounts Of Liabilities Presented In Consolidated Balance Sheet [Member]
Derivative [Line Items]
Liability Derivatives	67,111	72,126
Repurchase Agreements	305,251	302,577
Total Offsetting Financial Liabilities	372,362	374,703
Offsetting Of Financial Liabilities [Member] | Financial Instruments Gross Amounts Not Offset In Consolidated Balance Sheet [Member]
Derivative [Line Items]
Liability Derivatives	0	0
Repurchase Agreements	(305,251)	(302,577)
Total Offsetting Financial Liabilities	(305,251)	(302,577)
Offsetting Of Financial Liabilities [Member] | Cash Collateral Posted Gross Amounts Not Offset In Consolidated Balance Sheet [Member]
Derivative [Line Items]
Liability Derivatives	(67,920)	(75,103)
Repurchase Agreements	0	0
Total Offsetting Financial Liabilities	(67,920)	(75,103)
Offsetting Of Financial Liabilities [Member] | Net Amount [Member]
Derivative [Line Items]
Liability Derivatives	(809)	(2,977)
Repurchase Agreements	0	0
Total Offsetting Financial Liabilities	$ (809)	$ (2,977)