Commitments and Contingencies - Additional Information (Detail) £ in Millions	3 Months Ended	9 Months Ended		12 Months Ended				54 Months Ended
	Sep. 30, 2018 USD ($)	Sep. 30, 2018 USD ($)	Sep. 30, 2018 GBP (£)	Dec. 31, 2014 USD ($)	Dec. 31, 2013 USD ($)	Dec. 31, 2012 USD ($)	Dec. 31, 2011 USD ($) Hospital	Sep. 30, 2018 USD ($)	Dec. 31, 2017 USD ($)	Jan. 31, 2017 USD ($)	Jan. 31, 2017 GBP (£)	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)
Commitments and Contingencies Disclosure [Line Items]
Self-insured for professional and general liability, current	$ 48,000,000	$ 48,000,000						$ 48,000,000	$ 54,000,000
Compensation liability claims	71,000,000	71,000,000						71,000,000	70,000,000
Compensation and related benefits	37,000,000	37,000,000						37,000,000	35,000,000
Accounts receivable, net	1,543,348,000	1,543,348,000						1,543,348,000	1,500,898,000
Department of Human Services
Commitments and Contingencies Disclosure [Line Items]
Repayment of legal settlement amount on demand				$ 4,000,000	$ 4,000,000	$ 4,000,000	$ 4,000,000
Amount claimed from over payments of legal settlements				7,000,000	7,000,000	$ 4,000,000
Amount claimed from over payments of legal settlements due to change in calculations					2,000,000
McAllen Medical Center
Commitments and Contingencies Disclosure [Line Items]
Amount awarded to plaintiff		3,850,000
Total amount of order, including prejudgment interest		9,000,000
DOJ
Commitments and Contingencies Disclosure [Line Items]
Increase in aggregate pre-tax reserve		90,000,000
Pre-tax increase to reserve for civil aspects	48,000,000	70,000,000
River Point Behavioral Health
Commitments and Contingencies Disclosure [Line Items]
Amount withheld in contingent medicare payment suspension								9,000,000
Letters of Credit and Surety Bonds
Commitments and Contingencies Disclosure [Line Items]
Off-balance sheet contingent obligation	116,000,000	116,000,000						116,000,000
Illinois
Commitments and Contingencies Disclosure [Line Items]
Accounts receivable, net	24,000,000	24,000,000						24,000,000	25,000,000
Accounts receivable net greater than sixty days Past due	10,000,000	10,000,000						10,000,000	8,000,000
Pennsylvania | Department of Human Services
Commitments and Contingencies Disclosure [Line Items]
Number of behavioral health care hospitals | Hospital							7
Wind Storms
Commitments and Contingencies Disclosure [Line Items]
Minimum Insurance Deductible	50,000	50,000						50,000
Maximum insurance deductible	250,000	250,000						250,000
Flood
Commitments and Contingencies Disclosure [Line Items]
Maximum insurance deductible	500,000	500,000						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	107,000,000	107,000,000						107,000,000
Other Debt Guarantees | Letters of Credit and Surety Bonds
Commitments and Contingencies Disclosure [Line Items]
Off-balance sheet contingent obligation	9,000,000	9,000,000						9,000,000
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	$ 1,000,000,000						$ 1,000,000,000
Percentage of insurance deductible	5.00%	5.00%						5.00%
Maximum | Earthquake | LAS VEGAS, NEVADA
Commitments and Contingencies Disclosure [Line Items]
Commercial property insurance policies covering catastrophic losses	$ 500,000,000	$ 500,000,000						$ 500,000,000
Maximum | Earthquake | CALIFORNIA
Commitments and Contingencies Disclosure [Line Items]
Commercial property insurance policies covering catastrophic losses	130,000,000	130,000,000						130,000,000
Maximum | Earthquake | Faulty Zones of UNITED STATES
Commitments and Contingencies Disclosure [Line Items]
Commercial property insurance policies covering catastrophic losses	100,000,000	100,000,000						100,000,000
Maximum | Earthquake | PUERTO RICO
Commitments and Contingencies Disclosure [Line Items]
Commercial property insurance policies covering catastrophic losses	40,000,000	40,000,000						40,000,000
Maximum | Earthquake | OTHER STATES
Commitments and Contingencies Disclosure [Line Items]
Commercial property insurance policies covering catastrophic losses	$ 250,000,000	$ 250,000,000						$ 250,000,000
Minimum | Wind Storms
Commitments and Contingencies Disclosure [Line Items]
Percentage of insurance deductible	3.00%	3.00%						3.00%
General and Professional Liability
Commitments and Contingencies Disclosure [Line Items]
Self-insured for professional and general liability	$ 243,000,000	$ 243,000,000						$ 243,000,000	$ 229,000,000
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%	10.00%						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				$ 250,000,000	$ 200,000,000							$ 250,000,000	$ 250,000,000
Liability for claims paid under commercially insured coverage	$ 5,000,000	$ 5,000,000						$ 5,000,000