Stock Based Compensation Plans - Additional Information (Detail) (USD $) In Millions, except Share data, unless otherwise specified	12 Months Ended
	Dec. 31, 2012 Y	Dec. 31, 2011	Dec. 31, 2010	May 16, 2006
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Issuance of new shares under plan				3,000,000
Shares available for grants under the plan	1,351,319
Compensation cost	$ 12.8	$ 12.2	$ 11.1
Tax benefit related to compensation expense	4.5	4.3	3.9
Options granted vesting (in years)	4 years	4 years	4 years
Number of years options must be exercised from date of grant	10
Cash received for options exercised	2.3	3.9	12.1
Intrinsic value of options exercised	1.2	1.8	2.6
Excess tax expense realized from options exercised	0.4	1.5	0.1
Aggregate intrinsic value, outstanding	8.4
Aggregate intrinsic value, exercisable	6.5
Weighted average remaining contractual life, outstanding (in years)	5 years
Weighted average remaining contractual life, exercisable (in years)	2 years 8 months 12 days
Weighted average grant date fair value of options granted	$ 8.87	$ 12.23	$ 11.48
Unrecognized compensation expense related to unvested stock options	14.5
Unrecognized compensation expense recognized over a weighted average period (in years)	2 years 3 months 18 days
Performance metric for performance based restricted stock units	100.00%
Performance metric for performance based restricted stock units minimum potential range	0.00%
Performance metric for performance based restricted stock units maximum potential range	133.00%
Average period based on relative TSR (in years)	3
Performance-Based Shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Fair Value of Vested shares		1.3	1.3
Increase as grant in period, percentage	100.00%
Decrease as forfeited, percentage	100.00%
Aggregate intrinsic value of restricted stock units	5.1
Weighted average remaining contractual life (in years)	2 years 8 months 12 days
Performance And Market-Based Restricted Stock Units
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Market-based awards granted to senior managment	96,150	42,500	41,250
Performance metric, minimum	0.00%
Performance metric, maximum	150.00%
Restricted Stock Units (RSUs)
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Fair Value of Vested shares	1.6	10.2	5.0
Unrecognized compensation expense related to unvested stock options	6.0
Unrecognized compensation expense recognized over a weighted average period (in years)	2 years 7 months 6 days
Restricted Stock And Restricted Stock Unit
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Fair Value of Vested shares	1.7
Aggregate intrinsic value of restricted stock units	$ 29.1
Weighted average remaining contractual life (in years)	1 year 6 months
Granted 2009
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Weighted average grant date fair value of options granted		$ 34.19
Performance-based stock units included as forfeited due to completion levels		47,375
Granted 2007
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Weighted average grant date fair value of options granted			$ 48.46
Performance-based stock units included as forfeited due to completion levels			11,472
Granted In 2008
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Weighted average grant date fair value of options granted			$ 45.21
Performance-based stock units included as forfeited due to completion levels			25,055
Granted In Twenty Ten
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Performance-based stock units included as forfeited due to completion levels	31,750
Performance-based stock units, weighted average grant date fair value	$ 42.15
Senior management | Performance And Market-Based Restricted Stock Units
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Market-based awards granted to senior managment	92,400
Vesting Rate In The Third Year
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Options granted vesting rate	50	50	50
Vesting Rate In The Final Year
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Options granted vesting rate	50	50	50
Vesting Rate After Three Years | Restricted Stock Shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Options granted vesting (in years)	3 years	3 years	3 years
Options granted vesting rate	50	50	50
Vesting Rate After Three Years | Market Based Restricted Stock
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Options granted vesting (in years)	3 years
Options granted vesting rate	50
Vesting Rate After Three Years | Performance-Based Shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Options granted vesting (in years)		3 years	3 years
Options granted vesting rate		50	50
Vesting Rate After Four Years | Restricted Stock Shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Options granted vesting (in years)	4 years	4 years	4 years
Options granted vesting rate	50	50	50
Vesting Rate After Four Years | Market Based Restricted Stock
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Options granted vesting (in years)	4 years
Options granted vesting rate	50
Vesting Rate After Four Years | Performance-Based Shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Options granted vesting (in years)		4 years	4 years
Options granted vesting rate		50	50