Revenue Recognition - Disaggregation of Revenue by Segment (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Disaggregation of Revenue [Line Items]
Total operating revenues, net	$ 151,890	$ 160,370
Reportable Operating Segments [Member]
Disaggregation of Revenue [Line Items]
Total operating revenues, net	151,890	160,370
Reportable Operating Segments [Member] | Oil and Gas [Member]
Disaggregation of Revenue [Line Items]
Total operating revenues, net	84,946	95,640
Reportable Operating Segments [Member] | Air Medical [Member]
Disaggregation of Revenue [Line Items]
Total operating revenues, net	56,646	56,988
Reportable Operating Segments [Member] | Air Medical [Member] | Traditional Provider Model [Member]
Disaggregation of Revenue [Line Items]
Total operating revenues, net	11,018	11,106
Reportable Operating Segments [Member] | Air Medical [Member] | Independent Provider Model [Member]
Disaggregation of Revenue [Line Items]
Total operating revenues, net	45,628	45,882
Reportable Operating Segments [Member] | Technical Services [Member]
Disaggregation of Revenue [Line Items]
Total operating revenues, net	$ 10,298	$ 7,742