Summary of Significant Accounting Policies - Provisions for Contractual Discounts and Uncompensated Care Applied to Air Medical Trade Receivables Expressed as Percentage of Total Segment Accounts Receivable (Detail) - Air Medical [Member]	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Provisions For Contractual Discounts And Estimated Uncompensated Care [Line Items]
Allowance for contractual discounts	66.00%	67.00%	65.00%
Trade Accounts Receivable [Member]
Provisions For Contractual Discounts And Estimated Uncompensated Care [Line Items]
Allowance for contractual discounts	53.00%	56.00%
Allowance for uncompensated care	24.00%	23.00%