Basis of Presentation - Additional Information (Details) - USD ($) $ in Millions	9 Months Ended	12 Months Ended
	Sep. 30, 2018	Dec. 31, 2017
Disaggregation of Revenue [Line Items]
Typical contract length	30 days
Provisional income tax benefit related to remeasurement of deferred tax balances		$ 49.2
Minimum [Member]
Disaggregation of Revenue [Line Items]
Typical contract length	1 year
Accounts receivable invoice term	30 days
Contract termination notice period	30 days
Maximum [Member]
Disaggregation of Revenue [Line Items]
Typical contract length	7 years
Accounts receivable invoice term	60 days
Contract termination notice period	180 days
Oil and Gas [Member]
Disaggregation of Revenue [Line Items]
Receivables	$ 103.1	106.8
Air Medical [Member]
Disaggregation of Revenue [Line Items]
Receivables	91.8	74.4
Technical Services [Member]
Disaggregation of Revenue [Line Items]
Receivables	$ 2.5	$ 4.8