Fair Value Measurements - Reconciliation of the Net Beginning and Ending Balances for Derivative Instruments (Detail) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Unrealized gains included in derivative fair value	$ (58,895)	$ 40,312	$ 30,543
Fair Value, Inputs, Level 3
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Beginning balance
Unrealized gains included in derivative fair value	548
Ending balance	$ 548