Significant Accounting Policies - Narrative (Details) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018 USD ($) Project	Jun. 30, 2017 Project	Jun. 30, 2018 USD ($) Project Source	Jun. 30, 2017 Project	Dec. 31, 2017 USD ($)
Accounting Policies [Abstract]
Number of product revenue sources			2
Unbilled receivables | $	$ 44.7		$ 44.7		$ 41.0
Number of projects | Project	9	6	10	7
Percentage of total revenues	13.00%	16.00%	13.00%	14.00%
Percentage of product revenues	19.00%	20.00%	18.00%	18.00%
Number of service revenue sources			2