Supplemental Quarterly Financial Information (Parenthetical) (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Dec. 31, 2011	Jun. 30, 2011 Assets Held-for-sale
Quarterly Financial Information [Line Items]
Impairement of oil and gas properties	$ 75.2	$ 0.7	$ 28.6	$ 33.6	$ 71.1