Fair Value Measurements - Assets and Liabilities Measured at Fair Value on Recurring Basis (Detail) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Marketable securities (Other assets)	$ 8,425	$ 8,248
Assets, Fair Value, Total	161,937	14,175
Liabilities, Fair Value, Total		8,223
Derivative Contracts [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Assets, Fair Value	153,512	5,927
Derivative Liabilities, Fair Value		8,223
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Marketable securities (Other assets)	8,425	8,248
Assets, Fair Value, Total	8,425	8,248
Significant Other Observable Inputs (Level 2) [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets, Fair Value, Total	153,512	5,927
Liabilities, Fair Value, Total		8,223
Significant Other Observable Inputs (Level 2) [Member] | Derivative Contracts [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Assets, Fair Value	153,512	5,927
Derivative Liabilities, Fair Value		$ 8,223