Derivative Instruments - Derivative Liabilities (Detail) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Derivative [Line Items]
Net Amount Presented in the Balance Sheet	$ 46,160	$ 43,369
Derivative Liabilities Subject to Master Netting Arrangement [Member]
Derivative [Line Items]
Net Amount Presented in the Balance Sheet	45,985	40,323
Gross Amount Not Offset in the Balance Sheet Financial Instruments, Derivative Liabilities	30,976	36,581
Gross Amount Not Offset in the Balance Sheet Cash Collateral Pledged, Derivative Liabilities	13,243	1,610
Net Amount, Derivative Liabilities	1,766	2,132
Derivative Liabilities Subject to Master Netting Arrangement [Member] | Counterparty A [Member]
Derivative [Line Items]
Net Amount Presented in the Balance Sheet	3,897	4,934
Gross Amount Not Offset in the Balance Sheet Financial Instruments, Derivative Liabilities	3,897	4,934
Gross Amount Not Offset in the Balance Sheet Cash Collateral Pledged, Derivative Liabilities	0	0
Net Amount, Derivative Liabilities	0	0
Derivative Liabilities Subject to Master Netting Arrangement [Member] | Counterparty B [Member]
Derivative [Line Items]
Net Amount Presented in the Balance Sheet	1,741	3,249
Gross Amount Not Offset in the Balance Sheet Financial Instruments, Derivative Liabilities	1,741	3,249
Derivative Liabilities Subject to Master Netting Arrangement [Member] | Counterparty C [Member]
Derivative [Line Items]
Net Amount Presented in the Balance Sheet	1,124	1,431
Gross Amount Not Offset in the Balance Sheet Financial Instruments, Derivative Liabilities	1,124	1,431
Derivative Liabilities Subject to Master Netting Arrangement [Member] | Counterparty D [Member]
Derivative [Line Items]
Net Amount Presented in the Balance Sheet	6,757	9,614
Gross Amount Not Offset in the Balance Sheet Financial Instruments, Derivative Liabilities	6,757	9,614
Derivative Liabilities Subject to Master Netting Arrangement [Member] | Counterparty E [Member]
Derivative [Line Items]
Net Amount Presented in the Balance Sheet	3,444	6,257
Gross Amount Not Offset in the Balance Sheet Financial Instruments, Derivative Liabilities	3,444	6,257
Derivative Liabilities Subject to Master Netting Arrangement [Member] | Counterparty F [Member]
Derivative [Line Items]
Net Amount Presented in the Balance Sheet	895	13
Gross Amount Not Offset in the Balance Sheet Financial Instruments, Derivative Liabilities	875	13
Net Amount, Derivative Liabilities	20
Derivative Liabilities Subject to Master Netting Arrangement [Member] | Counterparty G [Member]
Derivative [Line Items]
Net Amount Presented in the Balance Sheet	7,173	5,309
Gross Amount Not Offset in the Balance Sheet Financial Instruments, Derivative Liabilities	7,173	5,309
Derivative Liabilities Subject to Master Netting Arrangement [Member] | Counterparty H [Member]
Derivative [Line Items]
Net Amount Presented in the Balance Sheet	1,746	2,257
Gross Amount Not Offset in the Balance Sheet Financial Instruments, Derivative Liabilities		125
Net Amount, Derivative Liabilities	1,746	2,132
Derivative Liabilities Subject to Master Netting Arrangement [Member] | Counterparty I [Member]
Derivative [Line Items]
Net Amount Presented in the Balance Sheet	5,965	5,649
Gross Amount Not Offset in the Balance Sheet Financial Instruments, Derivative Liabilities	5,965	5,649
Derivative Liabilities Subject to Master Netting Arrangement [Member] | Counterparty J [Member]
Derivative [Line Items]
Net Amount Presented in the Balance Sheet	13,243	1,610
Gross Amount Not Offset in the Balance Sheet Cash Collateral Pledged, Derivative Liabilities	$ 13,243	$ 1,610