Summary of Significant Accounting Policies - Additional Information (Detail)	12 Months Ended
	Dec. 31, 2017 USD ($) Aircraft	Dec. 31, 2016 USD ($) Aircraft	Dec. 31, 2015 USD ($) Aircraft
Summary Of Significant Accounting Policies [Line Items]
Percentage of stock owned by CEO	70.90%
Maturities Period of liquid investments	Three months or less
Valuation reserves related to obsolescence and slow moving inventory	$ 20,900,000	$ 17,300,000
Property and equipment – net	946,765,000	903,977,000
Impairments of assets	368,000	407,000	$ 0
Insurance retention per claim	250,000
Accrued liabilities related to health care claims	1,300,000	1,900,000
Amount deductible per incident	500,000
Accrued expense related to workers compensation claims	2,900,000	2,500,000
Amounts paid to offshore insurance captive	500,000	600,000
Allowance for contractual discounts against outstanding accounts receivable	117,800,000	111,900,000	103,600,000
Allowance for uncompensated care against outstanding accounts receivable	52,500,000	46,300,000	41,900,000
Value of uncompensated care services	7,100,000	8,800,000	9,300,000
Estimated cost of providing charity services	$ 1,600,000	1,900,000	2,000,000
Percentage of reserved category of self pay	100.00%
Provisional income tax benefit related to remeasurement of deferred tax balances	$ 49,200,000
Allowance for doubtful accounts	7,200,000	6,000,000
Maximum [Member]
Summary Of Significant Accounting Policies [Line Items]
Unrealized losses recorded	300,000
Light Aircraft [Member]
Summary Of Significant Accounting Policies [Line Items]
Impairments of assets	400,000	400,000.0	0
Oil and Gas [Member]
Summary Of Significant Accounting Policies [Line Items]
Cash proceeds	1,300,000	15,000,000	3,700,000
Gain (loss) on disposal of assets	$ 300,000	$ 3,300,000	$ 0
Oil and Gas [Member] | Light Aircraft [Member]
Summary Of Significant Accounting Policies [Line Items]
Number of aircraft sold | Aircraft		12	8
Number of Aircraft impaired | Aircraft	2
Carrying value of aircraft	$ 1,800,000
Oil and Gas [Member] | Light Aircraft [Member] | Fair Value, Inputs, Level 3 [Member]
Summary Of Significant Accounting Policies [Line Items]
Market value of aircraft	$ 1,400,000
Oil and Gas [Member] | Medium Aircraft [Member]
Summary Of Significant Accounting Policies [Line Items]
Number of aircraft sold | Aircraft	6	11	6
Oil and Gas [Member] | Fixed Wing Aircraft [Member]
Summary Of Significant Accounting Policies [Line Items]
Number of aircraft sold | Aircraft	1
Air Medical and Technical Services [Member]
Summary Of Significant Accounting Policies [Line Items]
Threshold customer percentage for operating revenue not exceeded during the year (percent)	2.00%	2.00%	2.00%
Air Medical [Member]
Summary Of Significant Accounting Policies [Line Items]
Allowance for contractual discounts against outstanding accounts receivable	$ 117,800,000	$ 111,900,000	$ 103,600,000
Allowance for uncompensated care against outstanding accounts receivable	$ 52,500,000	$ 46,300,000	41,900,000
Aircraft [Member] | Maximum [Member]
Summary Of Significant Accounting Policies [Line Items]
Salvage value of aircraft	54.00%
Aircraft [Member] | Minimum [Member]
Summary Of Significant Accounting Policies [Line Items]
Salvage value of aircraft	25.00%
Flight Equipment [Member] | Maximum [Member]
Summary Of Significant Accounting Policies [Line Items]
Estimated useful life	15 years
Flight Equipment [Member] | Minimum [Member]
Summary Of Significant Accounting Policies [Line Items]
Estimated useful life	5 years
Other Equipment [Member] | Maximum [Member]
Summary Of Significant Accounting Policies [Line Items]
Estimated useful life	10 years
Other Equipment [Member] | Minimum [Member]
Summary Of Significant Accounting Policies [Line Items]
Estimated useful life	3 years
Leasehold Improvements [Member] | Maximum [Member]
Summary Of Significant Accounting Policies [Line Items]
Estimated useful life	10 years
Leasehold Improvements [Member] | Minimum [Member]
Summary Of Significant Accounting Policies [Line Items]
Estimated useful life	6 years
Accounts Receivable [Member] | Customer Concentration Risk [Member]
Summary Of Significant Accounting Policies [Line Items]
Percentage owed by an overseas customer (percent)	10.70%
Aircraft [Member] | Oil and Gas [Member]
Summary Of Significant Accounting Policies [Line Items]
Property and equipment – net			3,700,000
Replacement Parts [Member] | Oil and Gas [Member]
Summary Of Significant Accounting Policies [Line Items]
Gain (loss) on disposal of assets			$ (300,000)
Two Largest Customers [Member] | Oil and Gas [Member]
Summary Of Significant Accounting Policies [Line Items]
Percentage of consolidated operating revenues	23.00%	24.00%	26.00%
Percentage of trade receivables	16.00%	16.00%
One Customer [Member]
Summary Of Significant Accounting Policies [Line Items]
Percentage of trade receivables	10.00%	13.00%
One Customer [Member] | Air Medical [Member]
Summary Of Significant Accounting Policies [Line Items]
Percentage of consolidated operating revenues		5.00%	9.00%