Summary of Significant Accounting Policies - Disaggregated Revenue by Segment (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Disaggregation of Revenue [Line Items]
Operating revenues, net	$ 176,023	$ 168,787	$ 169,243	$ 160,370	$ 148,336	$ 150,167	$ 146,424	$ 134,618	$ 674,423	$ 579,545	$ 634,098
Operating Segments [Member]
Disaggregation of Revenue [Line Items]
Operating revenues, net									674,423	579,545	634,098
Operating Segments [Member] | Oil and Gas [Member]
Disaggregation of Revenue [Line Items]
Operating revenues, net									380,238	298,398	324,129
Operating Segments [Member] | Air Medical [Member]
Disaggregation of Revenue [Line Items]
Operating revenues, net									257,132	257,273	281,868
Operating Segments [Member] | Technical Services [Member]
Disaggregation of Revenue [Line Items]
Operating revenues, net									37,053	23,874	$ 28,101
Traditional Provider Model [Member] | Operating Segments [Member] | Air Medical [Member]
Disaggregation of Revenue [Line Items]
Operating revenues, net									45,220	44,147
Independent Provider Model [Member] | Operating Segments [Member] | Air Medical [Member]
Disaggregation of Revenue [Line Items]
Operating revenues, net									$ 211,912	$ 213,126