Capital Stock - Common Stock Outstanding (Detail)	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Class of Stock [Line Items]
Balance, beginning of year	384,117,643	382,391,742	336,436,972
Shares issued for stock-based compensation plans:
Treasury shares issued	60,767	144,313	363,263
Common shares issued	1,189,693	1,581,588	1,864,498
Balance, end of year	391,640,770	384,117,643	382,391,742
Cordillera Energy Partners [Member]
Shares issued for stock-based compensation plans:
Stock issued during period, pursuant to acquisitions	6,272,667
Bp Permian Basin [Member]
Shares issued for stock-based compensation plans:
Stock issued during period, pursuant to acquisitions			26,450,000
Mariner Energy [Member]
Shares issued for stock-based compensation plans:
Stock issued during period, pursuant to acquisitions			17,277,009