Commitments and Contingencies - Additional Information (Detail) £ in Millions	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2020 USD ($)	Mar. 31, 2020 USD ($)	Sep. 30, 2020 USD ($)	Sep. 30, 2020 GBP (£)	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)	Dec. 31, 2013 USD ($)	Dec. 31, 2012 USD ($)	Dec. 31, 2011 USD ($)	Sep. 30, 2020 GBP (£)	Jun. 01, 2020 USD ($)	Jan. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)
Commitments and Contingencies Disclosure [Line Items]
Self-insured for professional and general liability, current	$ 50,000,000		$ 50,000,000										$ 42,000,000
Compensation liability claims	90,000,000		90,000,000										81,000,000
Compensation and related benefits	46,000,000		46,000,000										40,000,000
Department of Human Services
Commitments and Contingencies Disclosure [Line Items]
Aggregate payment made					$ 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
Per Occurrence/Per Location Deductible | Wind Storms
Commitments and Contingencies Disclosure [Line Items]
Commercial property insurance policies covering catastrophic losses											$ 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 | Flood
Commitments and Contingencies Disclosure [Line Items]
Commercial property insurance policies covering catastrophic losses	$ 100,000,000		$ 100,000,000
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 | PUERTO RICO | Flood
Commitments and Contingencies Disclosure [Line Items]
Commercial property insurance policies covering catastrophic losses	1,000,000		1,000,000
Maximum | OTHER STATES | Earthquake
Commitments and Contingencies Disclosure [Line Items]
Commercial property insurance policies covering catastrophic losses	250,000,000		250,000,000
Maximum | Texas | Flood
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	$ 20,000,000	$ 25,000,000
General and Professional Liability
Commitments and Contingencies Disclosure [Line Items]
Self-insured for professional and general liability	$ 262,000,000		$ 262,000,000										242,000,000
Subsidiaries | Professional Liability
Commitments and Contingencies Disclosure [Line Items]
Self-insured for professional and general liability													5,000,000	$ 5,000,000	$ 5,000,000
Purchased several excess policies through commercial insurance carriers per occurrence												$ 2,500,000
Purchased several excess policies through commercial insurance carriers per occurrence excess claim amount												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												10,000,000				$ 10,000,000
Subsidiaries | General Liability
Commitments and Contingencies Disclosure [Line Items]
Self-insured for professional and general liability													$ 3,000,000	$ 3,000,000	$ 3,000,000
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	$ 250,000,000	$ 200,000,000
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