Deferred Revenue - Additional Information (Detail) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Deferred Revenue [Line Items]
Deferred revenue current liabilities	$ 20.6	$ 29.2
Deferred revenue non-current liabilities	48.7	43.2
Deferred revenue recognized	8.8
Extended Transmission Coverage
Deferred Revenue [Line Items]
Deferred revenue current liabilities	20.3	20.4
Deferred revenue non-current liabilities	48.7	43.2
Deferred revenue recognized	20.8	21.0	19.9
U S Government Contracts
Deferred Revenue [Line Items]
Deferred revenue current liabilities	0.3	8.8	0.7
Deferred revenue recognized	$ 8.8