Commitments and Contingencies - Additional Information (Detail) £ in Millions	9 Months Ended		12 Months Ended	24 Months Ended
	Sep. 30, 2017 USD ($)	Sep. 30, 2017 GBP (£)	Dec. 31, 2011 USD ($) Hospital	Dec. 31, 2013 USD ($)	Jan. 31, 2017 USD ($)	Jan. 31, 2017 GBP (£)	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)
Commitments and Contingencies Disclosure [Line Items]
Self-insured for professional and general liability, current	$ 54,000,000						$ 48,000,000
Compensation liability claims	70,000,000						67,000,000
Compensation and related benefits	33,000,000						33,000,000
Accounts receivable, net	1,452,018,000						1,439,553,000
Department of Human Services
Commitments and Contingencies Disclosure [Line Items]
Repayment of legal settlement amount on demand			$ 4,000,000	$ 11,000,000
McAllen Medical Center
Commitments and Contingencies Disclosure [Line Items]
Amount awarded to plaintiff	3,850,000
Total amount of order, including prejudgment interest	8,500,000
Letters of Credit and Surety Bonds
Commitments and Contingencies Disclosure [Line Items]
Off-balance sheet contingent obligation	120,000,000
Location 1
Commitments and Contingencies Disclosure [Line Items]
Minimum Insurance Deductible	50,000
Location 2
Commitments and Contingencies Disclosure [Line Items]
Minimum Insurance Deductible	250,000
Illinois
Commitments and Contingencies Disclosure [Line Items]
Accounts receivable, net	52,000,000						38,000,000
Accounts receivable net greater than sixty days Past due	35,000,000						25,000,000
Pennsylvania | Department of Human Services
Commitments and Contingencies Disclosure [Line Items]
Number of behavioral health care hospitals | Hospital			7
Flood | Location 1
Commitments and Contingencies Disclosure [Line Items]
Maximum insurance deductible	500,000
Professional And General Liability
Commitments and Contingencies Disclosure [Line Items]
Increase in professional and general liability self-insurance reserves relating to prior years	15,000,000
Self Insurance Programs | Letters of Credit and Surety Bonds
Commitments and Contingencies Disclosure [Line Items]
Off-balance sheet contingent obligation	113,000,000
Other Debt Guarantees | Letters of Credit and Surety Bonds
Commitments and Contingencies Disclosure [Line Items]
Off-balance sheet contingent obligation	7,000,000
Cygnet Health Care Limited
Commitments and Contingencies Disclosure [Line Items]
Property insurance | £		£ 1,290
Maximum | Wind Storms
Commitments and Contingencies Disclosure [Line Items]
Commercial property insurance policies covering catastrophic losses	$ 1,000,000,000
Percentage of insurance deductible	5.00%
Maximum | Earthquake | LAS VEGAS, NEVADA
Commitments and Contingencies Disclosure [Line Items]
Commercial property insurance policies covering catastrophic losses	$ 500,000,000
Maximum | Earthquake | CALIFORNIA
Commitments and Contingencies Disclosure [Line Items]
Commercial property insurance policies covering catastrophic losses	130,000,000
Maximum | Earthquake | UNITED STATES
Commitments and Contingencies Disclosure [Line Items]
Commercial property insurance policies covering catastrophic losses	100,000,000
Maximum | Earthquake | PUERTO RICO
Commitments and Contingencies Disclosure [Line Items]
Commercial property insurance policies covering catastrophic losses	40,000,000
Maximum | Earthquake | OTHER STATES
Commitments and Contingencies Disclosure [Line Items]
Commercial property insurance policies covering catastrophic losses	$ 250,000,000
Minimum | Wind Storms
Commitments and Contingencies Disclosure [Line Items]
Percentage of insurance deductible	3.00%
General and Professional Liability
Commitments and Contingencies Disclosure [Line Items]
Self-insured for professional and general liability	$ 228,000,000						207,000,000
Subsidiaries | Professional Liability | Cygnet Health Care Limited
Commitments and Contingencies Disclosure [Line Items]
Self-insured for professional and general liability | £						£ 10
Subsidiaries | Professional Liability | Maximum
Commitments and Contingencies Disclosure [Line Items]
Self-insured for professional and general liability					$ 5,000,000		10,000,000
Subsidiaries | General Liability | Cygnet Health Care Limited
Commitments and Contingencies Disclosure [Line Items]
Self-insured for professional and general liability | £						£ 25
Subsidiaries | General Liability | Maximum
Commitments and Contingencies Disclosure [Line Items]
Self-insured for professional and general liability					$ 3,000,000		3,000,000
Subsidiaries | General And Professional Liability Insurance Policies
Commitments and Contingencies Disclosure [Line Items]
Percentage of liability for claims paid under commercially insured coverage	10.00%
Subsidiaries | General And Professional Liability Insurance Policies | Maximum
Commitments and Contingencies Disclosure [Line Items]
Purchased several excess policies through commercial insurance carriers per occurrence				$ 200,000,000			$ 250,000,000	$ 250,000,000	$ 250,000,000
Liability for claims paid under commercially insured coverage	$ 50,000,000