Commitments and Contingencies - Additional Information (Detail) £ in Millions	3 Months Ended		12 Months Ended			48 Months Ended
	Mar. 31, 2018 USD ($)	Mar. 31, 2018 GBP (£)	Dec. 31, 2013 USD ($)	Dec. 31, 2012 USD ($)	Dec. 31, 2011 USD ($) Hospital	Mar. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	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					$ 54,000,000	$ 54,000,000
Compensation liability claims	70,000,000					70,000,000	70,000,000
Compensation and related benefits	35,000,000					35,000,000	35,000,000
Accounts receivable, net	1,569,803,000					1,569,803,000	1,500,898,000
Department of Human Services
Commitments and Contingencies Disclosure [Line Items]
Repayment of legal settlement amount on demand			$ 11,000,000	$ 11,000,000	$ 4,000,000
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]
Pre-tax increase to reserve for civil aspects	13,000,000
Increase in aggregate pre-tax reserve	35,000,000
River Point Behavioral Health
Commitments and Contingencies Disclosure [Line Items]
Amount withheld in contingent medicare payment suspension						10,000,000
Letters of Credit and Surety Bonds
Commitments and Contingencies Disclosure [Line Items]
Off-balance sheet contingent obligation	117,000,000					117,000,000
Self Insurance Programs | Letters of Credit and Surety Bonds
Commitments and Contingencies Disclosure [Line Items]
Off-balance sheet contingent obligation	108,000,000					108,000,000
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
Illinois
Commitments and Contingencies Disclosure [Line Items]
Accounts receivable, net	35,000,000					35,000,000	25,000,000
Accounts receivable net greater than sixty days Past due	15,000,000					15,000,000	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
Maximum insurance deductible	250,000					250,000
Flood
Commitments and Contingencies Disclosure [Line Items]
Maximum insurance deductible	500,000					500,000
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
Percentage of insurance deductible	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
Maximum | Earthquake | CALIFORNIA
Commitments and Contingencies Disclosure [Line Items]
Commercial property insurance policies covering catastrophic losses	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
Maximum | Earthquake | PUERTO RICO
Commitments and Contingencies Disclosure [Line Items]
Commercial property insurance policies covering catastrophic losses	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
Minimum | Wind Storms
Commitments and Contingencies Disclosure [Line Items]
Percentage of insurance deductible	3.00%					3.00%
General and Professional Liability
Commitments and Contingencies Disclosure [Line Items]
Self-insured for professional and general liability	$ 232,000,000					$ 232,000,000	$ 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%
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	$ 5,000,000					$ 5,000,000