Derivative Instruments - Derivative Liabilities (Detail) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2014	Dec. 31, 2013
Derivative [Line Items]
Net Amount Presented in the Balance Sheet	$ 40,165	$ 43,369
Derivative Liabilities Subject to Master Netting Arrangement [Member]
Derivative [Line Items]
Net Amount Presented in the Balance Sheet	38,332	40,323
Gross Amount Not Offset in the Balance Sheet Financial Instruments, Derivative Liabilities	33,541	36,581
Gross Amount Not Offset in the Balance Sheet Cash Collateral Pledged, Derivative Liabilities	2,672	1,610
Net Amount, Derivative Liabilities	2,119	2,132
Derivative Liabilities Subject to Master Netting Arrangement [Member] | Counterparty A [Member]
Derivative [Line Items]
Net Amount Presented in the Balance Sheet	4,682	4,934
Gross Amount Not Offset in the Balance Sheet Financial Instruments, Derivative Liabilities	4,682	4,934
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	3,005	3,249
Gross Amount Not Offset in the Balance Sheet Financial Instruments, Derivative Liabilities	3,005	3,249
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	1,356	1,431
Gross Amount Not Offset in the Balance Sheet Financial Instruments, Derivative Liabilities	1,356	1,431
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	8,855	9,614
Gross Amount Not Offset in the Balance Sheet Financial Instruments, Derivative Liabilities	8,855	9,614
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	5,093	6,257
Gross Amount Not Offset in the Balance Sheet Financial Instruments, Derivative Liabilities	5,093	6,257
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	15	13
Gross Amount Not Offset in the Balance Sheet Financial Instruments, Derivative Liabilities	15	13
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 G [Member]
Derivative [Line Items]
Net Amount Presented in the Balance Sheet	4,804	5,309
Gross Amount Not Offset in the Balance Sheet Financial Instruments, Derivative Liabilities	4,804	5,309
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	2,119	2,257
Gross Amount Not Offset in the Balance Sheet Financial Instruments, Derivative Liabilities		125
Gross Amount Not Offset in the Balance Sheet Cash Collateral Pledged, Derivative Liabilities
Net Amount, Derivative Liabilities	2,119	2,132
Derivative Liabilities Subject to Master Netting Arrangement [Member] | Counterparty I [Member]
Derivative [Line Items]
Net Amount Presented in the Balance Sheet	5,731	5,649
Gross Amount Not Offset in the Balance Sheet Financial Instruments, Derivative Liabilities	5,731	5,649
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	2,672	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	2,672	1,610
Net Amount, Derivative Liabilities