Derivative Financial Instruments - Additional Information (Detail) (USD $)	12 Months Ended
	Mar. 31, 2013	Dec. 31, 2012	Mar. 31, 2012	Dec. 31, 2011
Derivative Instruments and Hedging Activities Disclosure [Line Items]
Net gain (loss) on derivative financial instruments	$ (5,820,000)	$ 11,735,000	$ (1,657,000)	$ 3,088,000
Net deferred gains expected to be reclassified from AOCI over next 12 months	4,000,000
Notional value of outstanding derivative contracts	1,600,000,000
Derivative contracts, maturity date	Dec. 31, 2017
Fair value of Derivative contracts	3,800,000
EPCI projects
Derivative Instruments and Hedging Activities Disclosure [Line Items]
Notional value of outstanding derivative contracts	$ 925,000,000