Business Segments and Geographic Areas - Schedule of Accounts Receivable and Revenue by Major Customers by Reporting Segments (Detail) - Segment	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Revenue, Major Customer [Line Items]
Number of reportable segments	3
Customer Concentration Risk [Member] | Accounts Receivable [Member]
Revenue, Major Customer [Line Items]
Concentration risk, percentage	10.70%
Customer Concentration Risk [Member] | Oil and Gas [Member] | Operating Revenues [Member] | Customer A [Member]
Revenue, Major Customer [Line Items]
Concentration risk, percentage	14.00%	14.00%	15.00%
Customer Concentration Risk [Member] | Oil and Gas [Member] | Operating Revenues [Member] | Customer B [Member]
Revenue, Major Customer [Line Items]
Concentration risk, percentage	9.00%	10.00%	11.00%
Customer Concentration Risk [Member] | Air Medical and Technical Services [Member] | Operating Revenues [Member] | Customer C [Member]
Revenue, Major Customer [Line Items]
Concentration risk, percentage	0.00%	5.00%	9.00%
Concentration of Credit Risk [Member] | Oil and Gas [Member] | Accounts Receivable [Member] | Customer A [Member]
Revenue, Major Customer [Line Items]
Concentration risk, percentage	11.00%	8.00%
Concentration of Credit Risk [Member] | Oil and Gas [Member] | Accounts Receivable [Member] | Customer B [Member]
Revenue, Major Customer [Line Items]
Concentration risk, percentage	5.00%	8.00%
Concentration of Credit Risk [Member] | Air Medical and Technical Services [Member] | Accounts Receivable [Member] | Customer C [Member]
Revenue, Major Customer [Line Items]
Concentration risk, percentage	10.00%	13.00%