Derivative Instruments - Derivative Liabilities (Detail) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2013	Dec. 31, 2012
Derivative [Line Items]
Net Amount Presented in the Balance Sheet	$ 43,369	$ 58,150
Derivative Liabilities Subject to Master Netting Arrangement [Member]
Derivative [Line Items]
Net Amount Presented in the Balance Sheet	40,323	58,134
Gross Amount Not Offset in the Balance Sheet Financial Instruments, Derivative Liabilities	36,581	55,901
Gross Amount Not Offset in the Balance Sheet Cash Collateral Pledged, Derivative Liabilities	1,610
Net Amount, Derivative Liabilities	2,132	2,233
Derivative Liabilities Subject to Master Netting Arrangement [Member] | Counterparty A [Member]
Derivative [Line Items]
Net Amount Presented in the Balance Sheet	4,934	8,393
Gross Amount Not Offset in the Balance Sheet Financial Instruments, Derivative Liabilities	4,934	8,393
Gross Amount Not Offset in the Balance Sheet Cash Collateral Pledged, Derivative Liabilities
Net Amount, Derivative Liabilities
Derivative Liabilities Subject to Master Netting Arrangement [Member] | Counterparty B [Member]
Derivative [Line Items]
Net Amount Presented in the Balance Sheet	3,249	5,601
Gross Amount Not Offset in the Balance Sheet Financial Instruments, Derivative Liabilities	3,249	5,601
Gross Amount Not Offset in the Balance Sheet Cash Collateral Pledged, Derivative Liabilities
Net Amount, Derivative Liabilities
Derivative Liabilities Subject to Master Netting Arrangement [Member] | Counterparty C [Member]
Derivative [Line Items]
Net Amount Presented in the Balance Sheet	1,431	2,145
Gross Amount Not Offset in the Balance Sheet Financial Instruments, Derivative Liabilities	1,431	2,145
Gross Amount Not Offset in the Balance Sheet Cash Collateral Pledged, Derivative Liabilities
Net Amount, Derivative Liabilities
Derivative Liabilities Subject to Master Netting Arrangement [Member] | Counterparty D [Member]
Derivative [Line Items]
Net Amount Presented in the Balance Sheet	9,614	12,354
Gross Amount Not Offset in the Balance Sheet Financial Instruments, Derivative Liabilities	9,614	12,354
Gross Amount Not Offset in the Balance Sheet Cash Collateral Pledged, Derivative Liabilities
Net Amount, Derivative Liabilities
Derivative Liabilities Subject to Master Netting Arrangement [Member] | Counterparty E [Member]
Derivative [Line Items]
Net Amount Presented in the Balance Sheet	6,257	8,846
Gross Amount Not Offset in the Balance Sheet Financial Instruments, Derivative Liabilities	6,257	8,846
Gross Amount Not Offset in the Balance Sheet Cash Collateral Pledged, Derivative Liabilities
Net Amount, Derivative Liabilities
Derivative Liabilities Subject to Master Netting Arrangement [Member] | Counterparty F [Member]
Derivative [Line Items]
Net Amount Presented in the Balance Sheet	13	353
Gross Amount Not Offset in the Balance Sheet Financial Instruments, Derivative Liabilities	13	282
Gross Amount Not Offset in the Balance Sheet Cash Collateral Pledged, Derivative Liabilities
Net Amount, Derivative Liabilities		71
Derivative Liabilities Subject to Master Netting Arrangement [Member] | Counterparty G [Member]
Derivative [Line Items]
Net Amount Presented in the Balance Sheet	5,309	5,497
Gross Amount Not Offset in the Balance Sheet Financial Instruments, Derivative Liabilities	5,309	5,497
Gross Amount Not Offset in the Balance Sheet Cash Collateral Pledged, Derivative Liabilities
Net Amount, Derivative Liabilities
Derivative Liabilities Subject to Master Netting Arrangement [Member] | Counterparty H [Member]
Derivative [Line Items]
Net Amount Presented in the Balance Sheet	2,257	3,937
Gross Amount Not Offset in the Balance Sheet Financial Instruments, Derivative Liabilities	125	1,775
Gross Amount Not Offset in the Balance Sheet Cash Collateral Pledged, Derivative Liabilities
Net Amount, Derivative Liabilities	2,132	2,162
Derivative Liabilities Subject to Master Netting Arrangement [Member] | Counterparty I [Member]
Derivative [Line Items]
Net Amount Presented in the Balance Sheet	5,649	11,008
Gross Amount Not Offset in the Balance Sheet Financial Instruments, Derivative Liabilities	5,649	11,008
Gross Amount Not Offset in the Balance Sheet Cash Collateral Pledged, Derivative Liabilities
Net Amount, Derivative Liabilities
Derivative Liabilities Subject to Master Netting Arrangement [Member] | Counterparty J [Member]
Derivative [Line Items]
Net Amount Presented in the Balance Sheet	1,610
Gross Amount Not Offset in the Balance Sheet Financial Instruments, Derivative Liabilities
Gross Amount Not Offset in the Balance Sheet Cash Collateral Pledged, Derivative Liabilities	1,610
Net Amount, Derivative Liabilities