Other Assets - Additional Information (Detail) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Finite And Infinite Lived Intangible Assets [Line Items]
Amortization expense	$ 17.5	$ 8.2	$ 6.6
Estimated future amortization expense, 2013	24.8
Estimated future amortization expense, 2014	19.5
Estimated future amortization expense, 2015	15.3
Estimated future amortization expense, 2016	12.6
Estimated future amortization expense, 2017	9.6
Recorded out-of-period adjustment attributable to purchase price allocation	1.5
Amount by which other assets increased	$ 4.4