Commitments and Contingencies - Additional Information (Detail) £ in Millions	3 Months Ended				9 Months Ended			12 Months Ended
	Sep. 30, 2021 USD ($)	Jun. 30, 2021 USD ($)	Sep. 30, 2020 USD ($)	Mar. 31, 2020 USD ($)	Sep. 30, 2021 USD ($)	Sep. 30, 2021 GBP (£)	Sep. 30, 2020 USD ($)	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)	Dec. 31, 2013 USD ($)	Dec. 31, 2012 USD ($)	Dec. 31, 2011 USD ($)	Sep. 30, 2021 GBP (£)	Dec. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2010 USD ($)
Commitments and Contingencies Disclosure [Line Items]
Self-insured for professional and general liability, current	$ 74,000,000				$ 74,000,000									$ 74,000,000
Compensation liability claims	110,000,000				110,000,000									105,000,000
Compensation and related benefits	55,000,000				55,000,000									55,000,000
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	$ 4,000,000
Amount claimed from over payments of legal settlements								7,000,000	8,000,000	7,000,000	4,000,000
Amount claimed from over payments of legal settlements due to change in calculations								3,000,000	2,000,000	2,000,000
Wind Storms
Commitments and Contingencies Disclosure [Line Items]
Maximum insurance deductible	2,500,000				2,500,000
Cygnet Health Care Limited
Commitments and Contingencies Disclosure [Line Items]
Property insurance | £						£ 1,500
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%								5.00%
Maximum | Earthquake | LAS VEGAS, NEVADA
Commitments and Contingencies Disclosure [Line Items]
Commercial property insurance policies covering catastrophic losses	$ 200,000,000				$ 200,000,000
Maximum | Earthquake | CALIFORNIA
Commitments and Contingencies Disclosure [Line Items]
Commercial property insurance policies covering catastrophic losses	150,000,000				150,000,000
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
Maximum | Flood
Commitments and Contingencies Disclosure [Line Items]
Commercial property insurance policies covering catastrophic losses	100,000,000				100,000,000
Maximum | Flood | PUERTO RICO
Commitments and Contingencies Disclosure [Line Items]
Commercial property insurance policies covering catastrophic losses	1,000,000				1,000,000
Maximum | Flood | Texas
Commitments and Contingencies Disclosure [Line Items]
Commercial property insurance policies covering catastrophic losses	$ 10,000,000				$ 10,000,000
Minimum | Wind Storms
Commitments and Contingencies Disclosure [Line Items]
Percentage of insurance deductible	3.00%				3.00%								3.00%
General And Professional Liability Insurance Policies
Commitments and Contingencies Disclosure [Line Items]
Self-insured for professional and general liability	$ 5,000,000	$ 36,000,000	$ 5,000,000	$ 20,000,000	$ 41,000,000		$ 25,000,000
General And Professional Liability Insurance Policies | Acute Care Hospital Services
Commitments and Contingencies Disclosure [Line Items]
Self-insured for professional and general liability					31,000,000		19,000,000
General And Professional Liability Insurance Policies | Behavioral Health Services
Commitments and Contingencies Disclosure [Line Items]
Self-insured for professional and general liability					10,000,000		6,000,000
General and Professional Liability
Commitments and Contingencies Disclosure [Line Items]
Self-insured for professional and general liability	340,000,000				340,000,000									264,000,000
Subsidiaries | Professional Liability
Commitments and Contingencies Disclosure [Line Items]
Purchased several excess policies through commercial insurance carriers per occurrence			2,500,000				2,500,000
Purchased several excess policies through commercial insurance carriers per occurrence excess claim amount			10,000,000				10,000,000
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	20,000,000		10,000,000		20,000,000		10,000,000								$ 5,000,000	$ 5,000,000	$ 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		3,000,000		$ 3,000,000								3,000,000	3,000,000	3,000,000	3,000,000
Subsidiaries | General And Professional Liability Insurance Policies
Commitments and Contingencies Disclosure [Line Items]
Purchased several excess policies through commercial insurance carriers per occurrence	$ 155,000,000				$ 155,000,000
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	$ 250,000,000	$ 200,000,000	$ 200,000,000	$ 200,000,000		$ 250,000,000	$ 250,000,000	$ 250,000,000	$ 250,000,000	$ 250,000,000	$ 200,000,000