Fair Value Measurements - Additional Information (Detail) (USD $)	Jun. 30, 2013	Dec. 31, 2012
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Minimum range of DOE index curve used in DCF model	$ 3.75	$ 3.68
Maximum range of DOE index curve used in DCF model	$ 3.81	$ 3.87
Weighted average DOE index curve used in DCF model	$ 3.78	$ 3.77