Relationship with Universal Health Realty Income Trust and Related Party Transactions - Additional Information (Detail)	3 Months Ended			9 Months Ended		12 Months Ended
	Sep. 30, 2021 USD ($) Hospital Facility	Sep. 30, 2020 USD ($) Hospital	Dec. 31, 2013	Sep. 30, 2021 USD ($) Hospital Bed Lease Facility	Sep. 30, 2020 USD ($) Hospital	Dec. 31, 2020 USD ($)	Dec. 31, 2019	Dec. 31, 2018
Related Party Transaction [Line Items]
Net revenues	$ 3,155,999,000	$ 2,912,541,000		$ 9,366,866,000	$ 8,471,962,000
Number of hospital facilities | Hospital				3
Chief Executive Officer
Related Party Transaction [Line Items]
Estimated payments to acquire life insurance policies				$ 28,000,000
Payments to acquire life insurance policies, net				1,000,000.0		$ 1,100,000
Chief Executive Officer | Trust Owned by CEO
Related Party Transaction [Line Items]
Estimated payments to acquire life insurance policies				$ 9,000,000
Minimum
Related Party Transaction [Line Items]
Lease initial terms	5 years			5 years
Minimum | Chief Executive Officer
Related Party Transaction [Line Items]
Estimated death benefit proceeds				$ 37,000,000
Maximum
Related Party Transaction [Line Items]
Lease initial terms	10 years			10 years
Relationship with Universal Health Realty Income Trust
Related Party Transaction [Line Items]
Trust outstanding shares held, percentage	5.70%			5.70%
Percentage of advisory fee on average invested real estate assets				0.70%	0.70%	0.70%	0.70%	0.70%
Pre-tax share of income from the Trust	$ 276,000	300,000		$ 977,000	$ 800,000
Dividends received from the Trust	552,000	543,000		1,700,000	1,600,000
Carrying value of investment in Trust	4,700,000			4,700,000		$ 5,400,000
Market value of investment in Trust	43,500,000			$ 43,500,000		50,600,000
Lease renewal period, years				5 years
Rent expense under operating leases	$ 4,000,000	$ 4,000,000		$ 13,000,000	$ 12,000,000
Notice period on renewal of lease				90 days
Period of rights of refusal to leased facilities				180 days
Number of hospital facilities | Hospital	3	3		3	3
Number of free-standing emergency departments to be acquired | Facility	2			2
Relationship with Universal Health Realty Income Trust | Behavioral Health Care Facility
Related Party Transaction [Line Items]
Lease initial terms	20 years			20 years
Lease renewal period, years				10 years
Number of triple net lease | Lease				5
Relationship with Universal Health Realty Income Trust | Clive, Iowa. | Behavioral Health Care Facility
Related Party Transaction [Line Items]
Number of bed available in behavioral health care facility | Bed				100
Relationship with Universal Health Realty Income Trust | Minimum
Related Party Transaction [Line Items]
Lease initial terms	13 years			13 years
Relationship with Universal Health Realty Income Trust | Minimum | Limited Liability Companies
Related Party Transaction [Line Items]
Non-controlling ownership interests by subsidiaries	95.00%			95.00%
Relationship with Universal Health Realty Income Trust | Maximum
Related Party Transaction [Line Items]
Lease initial terms	15 years			15 years
Relationship with Universal Health Realty Income Trust | Maximum | Limited Liability Companies
Related Party Transaction [Line Items]
Non-controlling ownership interests by subsidiaries	100.00%			100.00%
Relationship with Universal Health Realty Income Trust | Advisory Fee
Related Party Transaction [Line Items]
Net revenues	$ 1,100,000	$ 1,000,000.0		$ 3,300,000	$ 3,100,000
Premier, Inc. | Group Purchasing Organization Agreement | Restricted Stock
Related Party Transaction [Line Items]
Shares vesting period			7 years
Shares vesting period start year			2014
Shares vesting period end year			2020
Market value of retained vested shares				87,000,000		$ 78,000,000
Premier, Inc. | Other (Income) Expense, Net
Related Party Transaction [Line Items]
Dividend	$ 400,000	$ 400,000		$ 1,300,000	$ 400,000