Schedule of Basic Weighted Average Shares Reconciles to Diluted Weighted Average Shares Outstanding (Detail) In Thousands, unless otherwise specified	3 Months Ended									12 Months Ended
	Dec. 31, 2012		Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Weighted Average Number of Shares Outstanding [Line Items]
Basic weighted average common shares outstanding	82,296	[1]	82,482	83,180	82,804	82,596	82,402	82,151	81,430	82,689	82,145	80,466
Dilutive effect of stock options, DCUs and unvested shares										952	1,398	1,517
Diluted weighted average common shares outstanding	82,296	[1]	83,370	84,090	83,902	83,573	83,586	83,778	83,248	83,641	83,543	81,983

[1] Fourth quarter 2012 includes a $198.5 million asset impairment charge.