Derivative Instruments - Offsetting of Derivative Liabilities (Detail) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2013	Dec. 31, 2012
Derivative [Line Items]
Gross Amount	$ 44,422	$ 58,150
Gross Amounts Offset in the Balance Sheet, Derivative Liabilities
Net Amount Presented in the Balance Sheet	44,422	58,150
Derivative Liabilities Subject to Master Netting Arrangement [Member] | Interest Rate Contracts [Member]
Derivative [Line Items]
Gross Amount	42,931	58,134
Gross Amounts Offset in the Balance Sheet, Derivative Liabilities
Net Amount Presented in the Balance Sheet	42,931	58,134
Derivative Liabilities Not Subject to Master Netting Arrangement [Member] | Interest Rate Contracts [Member]
Derivative [Line Items]
Gross Amount	1,463
Gross Amounts Offset in the Balance Sheet, Derivative Liabilities
Net Amount Presented in the Balance Sheet	1,463
Derivative Liabilities Not Subject to Master Netting Arrangement [Member] | Equity Contracts [Member]
Derivative [Line Items]
Gross Amount	28	16
Gross Amounts Offset in the Balance Sheet, Derivative Liabilities
Net Amount Presented in the Balance Sheet	$ 28	$ 16