Summary of Significant Accounting Policies - Schedule of Provisions for Contractual Discounts and Estimated Uncompensated Care Applied to Air Medical Revenues Expressed as Percentage of Total Independent Provider Model Billings (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]
Provision for contractual discounts	66.00%	67.00%	65.00%
Provision for uncompensated care	7.00%	6.00%	8.00%