Property and Equipment - Summary of Company's Property and Equipment (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Property, Plant and Equipment [Line Items]
Property and equipment, gross	$ 1,377,260	$ 1,287,546
Less accumulated depreciation and amortization	(430,495)	(383,569)
Property and equipment, net	946,765	903,977
Flight Equipment [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, gross	1,225,205	1,144,315
Facility & Improvements [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, gross	64,316	61,187
Operating Equipment [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, gross	29,913	25,966
Data Processing Equipment [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, gross	34,747	33,881
Vehicles [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, gross	8,202	8,567
Medical Equipment [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, gross	8,354	7,963
Other [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, gross	$ 6,523	$ 5,667