Shareholders' Equity - Additional Information (Detail) (USD $) In Millions, except Per Share data, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Shareholders Equity [Line Items]
Date of dividend declared	Mar. 24, 2000
Rights Agreement expired year	March 2010
Cash dividends declared per share	$ 1.30	$ 1.16	$ 1.04
Dividends payable		$ 25.1	$ 21.7
Dividend per share declared		$ 0.31	$ 0.27
Dividend paid in cash per share	$ 0.34
Dividend declaration date	2012-12
Dividend payable date	2012-12
Shares authorized under repurchase program	250
Additional shares authorized under repurchase program	$ 3.8
Shares outstanding	4.70%
Share repurchase program, plan expiration date	Dec. 31, 2014