Cost of Contracts, net - Amortization Expense Related Cost of Contracts (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Deferred Costs, Capitalized, Prepaid, and Other Assets Disclosure [Abstract]
Amortization expense	$ 3.2	$ 3.4	$ 3.1
Weighted average life (years)	9 years 9 months 18 days	9 years 7 months 6 days	9 years