Commitments and Contingencies - Additional Information (Detail) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2013 USD ($) Contract	Mar. 31, 2013 AUD Contract
Commitments And Contingencies Disclosure [Abstract]
Accrued liability for legal contingencies	$ 14
Long term contracts unenforceable as penalty	2	2
Approximate percentage of natural gas volumes sold under long-term contracts that have liquidated damages provisions	90.00%	90.00%
Maximum expected contractual liquidated damages under long term contracts		50
Maximum Previous expected contractual liquidated damages under long term contracts		200
Undiscounted reserve for environmental remediation	$ 100