Derivative Instruments - Derivative Assets (Detail) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2013	Dec. 31, 2012
Derivative [Line Items]
Net Amount Presented in the Balance Sheet	$ 38,042	$ 58,008
Gross Amounts Not Offset in the Balance Sheet Financial Instruments, Derivative Assets	38,042	58,008
Gross Amounts Not Offset in the Balance Sheet Cash Collateral Received, Derivative Assets	397
Net Amount, Derivative Assets
Derivative Assets Subject to Master Netting Arrangement [Member]
Derivative [Line Items]
Net Amount Presented in the Balance Sheet	2,243
Gross Amounts Not Offset in the Balance Sheet Financial Instruments, Derivative Assets	1,846
Counterparty B [Member]
Derivative [Line Items]
Gross Amounts Not Offset in the Balance Sheet Cash Collateral Received, Derivative Assets
Net Amount, Derivative Assets
Counterparty B [Member] | Derivative Assets Subject to Master Netting Arrangement [Member]
Derivative [Line Items]
Net Amount Presented in the Balance Sheet	2
Gross Amounts Not Offset in the Balance Sheet Financial Instruments, Derivative Assets	2
Counterparty D [Member]
Derivative [Line Items]
Gross Amounts Not Offset in the Balance Sheet Cash Collateral Received, Derivative Assets
Net Amount, Derivative Assets
Counterparty D [Member] | Derivative Assets Subject to Master Netting Arrangement [Member]
Derivative [Line Items]
Net Amount Presented in the Balance Sheet	205
Gross Amounts Not Offset in the Balance Sheet Financial Instruments, Derivative Assets	205
Counterparty E [Member]
Derivative [Line Items]
Gross Amounts Not Offset in the Balance Sheet Cash Collateral Received, Derivative Assets
Net Amount, Derivative Assets		16
Counterparty E [Member] | Derivative Assets Subject to Master Netting Arrangement [Member]
Derivative [Line Items]
Net Amount Presented in the Balance Sheet	936	16
Gross Amounts Not Offset in the Balance Sheet Financial Instruments, Derivative Assets	936
Counterparty F [Member]
Derivative [Line Items]
Gross Amounts Not Offset in the Balance Sheet Cash Collateral Received, Derivative Assets
Net Amount, Derivative Assets
Counterparty F [Member] | Derivative Assets Subject to Master Netting Arrangement [Member]
Derivative [Line Items]
Net Amount Presented in the Balance Sheet	205
Gross Amounts Not Offset in the Balance Sheet Financial Instruments, Derivative Assets	205
Counterparty G [Member]
Derivative [Line Items]
Gross Amounts Not Offset in the Balance Sheet Cash Collateral Received, Derivative Assets
Net Amount, Derivative Assets
Counterparty G [Member] | Derivative Assets Subject to Master Netting Arrangement [Member]
Derivative [Line Items]
Net Amount Presented in the Balance Sheet	112
Gross Amounts Not Offset in the Balance Sheet Financial Instruments, Derivative Assets	112
Counterparty I [Member]
Derivative [Line Items]
Gross Amounts Not Offset in the Balance Sheet Cash Collateral Received, Derivative Assets
Net Amount, Derivative Assets
Counterparty I [Member] | Derivative Assets Subject to Master Netting Arrangement [Member]
Derivative [Line Items]
Net Amount Presented in the Balance Sheet	386
Gross Amounts Not Offset in the Balance Sheet Financial Instruments, Derivative Assets	386
Counterparty J [Member]
Derivative [Line Items]
Gross Amounts Not Offset in the Balance Sheet Cash Collateral Received, Derivative Assets	397
Net Amount, Derivative Assets
Counterparty J [Member] | Derivative Assets Subject to Master Netting Arrangement [Member]
Derivative [Line Items]
Net Amount Presented in the Balance Sheet	397
Gross Amounts Not Offset in the Balance Sheet Financial Instruments, Derivative Assets