Goodwill and Acquired Intangibles - Additional Information (Detail) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Intangible Assets And Goodwill [Line Items]
Amortization expense of acquired intangibles	$ 1.7	$ 1.5	$ 1.1
Accumulated amortization of intangibles	27.5	25.8
Maximum
Intangible Assets And Goodwill [Line Items]
Expected amortization expense in 2013	1.7
Expected amortization expense in 2014	1.7
Expected amortization expense in 2015	1.7
Expected amortization expense in 2016	1.7
Expected amortization expense in 2017	$ 1.7