Significant Accounting Policies - Summary of Trade Receivables (Details) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Accounts, Notes, Loans and Financing Receivable [Line Items]
Trade receivables	$ 173.6	$ 203.7
Allowance for doubtful accounts	(1.3)	(1.9)	$ (2.2)	$ (2.5)
Trade receivables, net	172.3	201.8
Trade receivables — billed
Accounts, Notes, Loans and Financing Receivable [Line Items]
Trade receivables	136.6	159.6
Trade receivables — unbilled
Accounts, Notes, Loans and Financing Receivable [Line Items]
Trade receivables	$ 37.0	$ 44.1