Business Segments and Geographic Areas - Summary of Gains on Disposition of Property and Equipment and Other Income (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Segment Reporting, Asset Reconciling Item [Line Items]
Expenditures for long-lived assets	$ 58,478	$ 82,871	$ 56,575
Depreciation and amortization	68,027	65,743	71,618
Total assets	1,401,846	1,448,440
Reportable Operating Segments [Member] | Oil and Gas [Member]
Segment Reporting, Asset Reconciling Item [Line Items]
Expenditures for long-lived assets	52,525	59,278	32,501
Depreciation and amortization	39,655	40,170	42,709
Total assets	722,734	674,788
Reportable Operating Segments [Member] | Air Medical [Member]
Segment Reporting, Asset Reconciling Item [Line Items]
Expenditures for long-lived assets	5,696	22,429	22,685
Depreciation and amortization	20,413	19,716	18,177
Total assets	372,646	368,474
Reportable Operating Segments [Member] | Technical Services [Member]
Segment Reporting, Asset Reconciling Item [Line Items]
Depreciation and amortization	581	564	518
Total assets	5,979	9,568
Corporate [Member]
Segment Reporting, Asset Reconciling Item [Line Items]
Expenditures for long-lived assets	257	1,164	1,389
Depreciation and amortization	7,378	5,293	$ 10,214
Total assets	$ 300,487	$ 395,610