Accounts Receivable and Major Customers - Additional Information (Detail) (USD $) In Millions, unless otherwise specified	9 Months Ended									12 Months Ended
	Sep. 30, 2013 Sales Revenue Customer Concentration Risk Customer	Sep. 30, 2012 Sales Revenue Customer Concentration Risk Customer	Sep. 30, 2013 Sales Revenue Customer Concentration Risk Major Customer 1	Sep. 30, 2012 Sales Revenue Customer Concentration Risk Major Customer 1	Sep. 30, 2013 Sales Revenue Customer Concentration Risk Major Customer 2	Sep. 30, 2012 Sales Revenue Customer Concentration Risk Major Customer 2	Sep. 30, 2013 Sales Revenue Customer Concentration Risk Major Customer 3	Sep. 30, 2012 Sales Revenue Customer Concentration Risk Major Customer 3	Sep. 30, 2013 Accounts Receivable Credit Concentration Risk	Dec. 31, 2012 Accounts Receivable Credit Concentration Risk
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of major customers	3	3
Revenues, major customers	$ 124.1	$ 115.6	$ 51.9	$ 45.9	$ 37.7	$ 39.5	$ 34.5	$ 30.3
Concentration risk, percentage	40.00%	49.00%	17.00%	19.00%	12.00%	17.00%	11.00%	13.00%	55.00%	37.00%
Accounts receivable, major customers									$ 46.1	$ 16.3