Offsetting Derivatives Assets (Detail) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2013	Dec. 31, 2012
Offsetting Assets [Line Items]
Gross Amounts of Recognized Assets, Total	$ 4,577	$ 68,361
Gross Amounts Offset in the Balance Sheet, Total	4,577	68,361
Net Amounts of Assets Presented in the Balance Sheet, Derivatives
Gross Amounts Not Offset in the Balance Sheet, Financial Instruments, Derivatives
Gross Amounts Not Offset in the Balance Sheet, Collateral Received, Derivatives
Net Amount, Derivatives
Derivatives
Offsetting Assets [Line Items]
Gross Amounts of Recognized Assets, Derivatives	4,577	68,361
Gross Amounts of Recognized Assets, Total	4,577	68,361
Gross Amounts Offset in the Balance Sheet, Total	4,577	68,361
Net Amounts of Assets Presented in the Balance Sheet, Derivatives
Gross Amounts Not Offset in the Balance Sheet, Financial Instruments, Derivatives
Gross Amounts Not Offset in the Balance Sheet, Collateral Received, Derivatives
Net Amount, Derivatives