Commitments and Contingencies - Additional Information (Detail) £ in Millions	3 Months Ended			12 Months Ended						60 Months Ended
	Mar. 31, 2019 USD ($)	Mar. 31, 2019 GBP (£)	Mar. 31, 2018 USD ($)	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2014 USD ($)	Dec. 31, 2013 USD ($)	Dec. 31, 2012 USD ($)	Dec. 31, 2011 USD ($) Hospital	Mar. 31, 2019 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	$ 42,000,000			$ 42,000,000						$ 42,000,000
Compensation liability claims	71,000,000			72,000,000						71,000,000
Compensation and related benefits	40,000,000			40,000,000						40,000,000
Accounts receivable, net	1,602,405,000			1,509,909,000						1,602,405,000
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
DOJ
Commitments and Contingencies Disclosure [Line Items]
Increase in aggregate pre-tax reserve	123,000,000			123,000,000	$ 22,000,000
Pre-tax increase to reserve for civil aspects			$ 13,000,000	102,000,000
River Point Behavioral Health
Commitments and Contingencies Disclosure [Line Items]
Amount withheld in contingent medicare payment suspension										9,000,000
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
Illinois
Commitments and Contingencies Disclosure [Line Items]
Accounts receivable, net	34,000,000			32,000,000						34,000,000
Accounts receivable net greater than sixty days Past due	19,000,000			18,000,000						19,000,000
Pennsylvania | Department of Human Services
Commitments and Contingencies Disclosure [Line Items]
Number of behavioral health care hospitals | Hospital									7
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 | LAS VEGAS, NEVADA | Earthquake
Commitments and Contingencies Disclosure [Line Items]
Commercial property insurance policies covering catastrophic losses	$ 500,000,000									$ 500,000,000
Maximum | CALIFORNIA | Earthquake
Commitments and Contingencies Disclosure [Line Items]
Commercial property insurance policies covering catastrophic losses	130,000,000									130,000,000
Maximum | Faulty Zones of UNITED STATES | Earthquake
Commitments and Contingencies Disclosure [Line Items]
Commercial property insurance policies covering catastrophic losses	100,000,000									100,000,000
Maximum | PUERTO RICO | Earthquake
Commitments and Contingencies Disclosure [Line Items]
Commercial property insurance policies covering catastrophic losses	40,000,000									40,000,000
Maximum | OTHER STATES | Earthquake
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	$ 246,000,000			$ 243,000,000						$ 246,000,000
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						$ 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
Subsidiaries | General And Professional Liability Insurance Policies | Maximum | Located in U.K.
Commitments and Contingencies Disclosure [Line Items]
Liability for claims paid under commercially insured coverage	$ 8,500,000									$ 8,500,000