Revenue Recognition - Disaggregation of Revenue by Segment (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Disaggregation of Revenue [Line Items]
Operating revenues, net	$ 165,172	$ 169,243	$ 317,062	$ 329,607
Reportable Operating Segments [Member]
Disaggregation of Revenue [Line Items]
Operating revenues, net	165,172	169,243	317,062	329,607
Reportable Operating Segments [Member] | Oil and Gas [Member]
Disaggregation of Revenue [Line Items]
Operating revenues, net	90,023	97,068	174,969	192,702
Reportable Operating Segments [Member] | Air Medical [Member]
Disaggregation of Revenue [Line Items]
Operating revenues, net	69,507	67,151	126,153	124,139
Reportable Operating Segments [Member] | Air Medical [Member] | Traditional Provider Model [Member]
Disaggregation of Revenue [Line Items]
Operating revenues, net	12,401	12,085	24,378	23,956
Reportable Operating Segments [Member] | Air Medical [Member] | Independent Provider Model [Member]
Disaggregation of Revenue [Line Items]
Operating revenues, net	57,106	55,066	101,775	100,183
Reportable Operating Segments [Member] | Technical Services [Member]
Disaggregation of Revenue [Line Items]
Operating revenues, net	$ 5,642	$ 5,024	$ 15,940	$ 12,766