Stock option activity (Detail) (Stock Option [Member], USD $) In Thousands, except Share data, unless otherwise specified	12 Months Ended
	Dec. 31, 2011
Stock Option [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares Outstanding at December 31, 2010	3,357,000
Shares Granted	216,000
Shares Exercised	(657,000)
Shares Expired	(1,439,000)
Shares Outstanding at December 31, 2011	1,477,000
Shares Exercisable at December 31, 2011	947,000
Weighted Average Exercise Price of Shares Outstanding at December 31, 2010	$ 22.58
Weighted Average Exercise Price of Shares Granted	$ 26.29
Weighted Average Exercise Price of Shares Exercised	$ 17.62
Weighted Average Exercise Price of Shares Expired	$ 27.91
Weighted Average Exercise Price of Shares Outstanding at December 31, 2011	$ 20.13
Weighted Average Exercise Price of Exercisable Shares at December 31, 2011	$ 19.89
Weighted Average Remaining Contractual Term of Outstanding Options at December 31, 2011	5.0
Weighted Average Remaining Contractual Term of Exercisable Option at December 31, 2011	4.4
Options Outstanding, Intrinsic Value at December 31, 2011	$ 3,811
Options, Exercisable, Intrinsic Value at December 31, 2011	$ 2,414