Share-based compensation plans - Schedule of Share-Based Compensation by Line Item in Consolidated Statements of Operations (Detail) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Allocated share based compensation expense	$ 6,303	$ 6,648	$ 8,138
Cost of sales [Member]
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Allocated share based compensation expense	592	153	210
Sales and marketing [Member]
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Allocated share based compensation expense	1,550	2,031	3,505
General and administrative [Member]
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Allocated share based compensation expense	4,023	4,322	4,105
Research and development [Member]
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Allocated share based compensation expense	$ 138	$ 142	$ 318