Selected Quarterly Financial Data (Parenthetical) (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended			9 Months Ended	12 Months Ended
	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Sep. 30, 2011	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Quarterly Financial Data [Line Items]
Favorable adjustment upon review of uncertain tax		$ 0.90	$ 3.20		$ 0.24	$ 3.50	$ 1.28
Effective tax rate reconciliation resulting from certain adjustments					22.80%	23.00%	16.60%
Reversal of tax reserve due to reversal of previous position	0.30	0	0.30	2.60	0.24	3.46
Reversal from reconciliation of tax returns to book provision	1.2					1.2	0.5
Reversal of tax reserve due to reversal of a valuation allowance	0.6			0.6
Increase from update to apportionment methodology						1.3
Prior Years
Quarterly Financial Data [Line Items]
Increase from update to apportionment methodology						$ 0.8