Deferred Compensation - Additional information (Detail) (USD $) In Millions, except Share data, unless otherwise specified	3 Months Ended			12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Investments And Employee Deferred Compensation Plan [Line Items]
Deferred Equity Participation Plan, distributions to key executives, age				Age 62 or upon or after their actual retirement
Compensation expenses				$ 1.2	$ 1.3	$ 1.4
Common stock distributed to employees under deferred compensation				19,000	17,000	40,000
Aggregate fair value of common stock issued				0.7	0.5	1.0
Unearned deferred compensation, value				5.6	6.8
Unearned deferred compensation, shares				610,000	629,000
Total intrinsic value of unvested common stock				21.1	21.0
Compensation approved by committee, value	7.3	6.5	5.9
Fair value of the funded cash award assets				41.6	28.6
Charge to compensation expenses related to awards				4.2	3.3	2.7
Cash and equity awards with aggregate fair value vested and distributed to participants				$ 0.7	$ 0.5	$ 1.1