Subsequent Events - Additional Information (Detail)	Nov. 01, 2017 USD ($) Facility Bed	Oct. 13, 2017 USD ($)	Sep. 15, 2017 USD ($)	Dec. 31, 2017 USD ($)	Sep. 30, 2017 USD ($)	Dec. 31, 2016 USD ($)
Subsequent Event [Line Items]
Unamortized debt issuance costs					$ 8,290,000	$ 9,831,000
Aggregate purchase price paid			$ 8,700,000
Forecast | 4.75% Senior Notes Due October 15, 2027
Subsequent Event [Line Items]
Unamortized debt issuance costs				$ 4,100,000
Revolving Credit Facility | $900.0 Million Revolving Credit Facility
Subsequent Event [Line Items]
Line of credit facility maximum borrowing capacity					$ 900,000,000	$ 900,000,000
Subsequent Event | Time to Change, Inc.
Subsequent Event [Line Items]
Aggregate purchase price paid	$ 13,200,000
Additional contingent consideration current	$ 9,000,000
Number of facility | Facility	3
Number of beds at the facility | Bed	422
Subsequent Event | 4.75% Senior Notes Due October 15, 2027
Subsequent Event [Line Items]
Aggregate principal amount		$ 250,000,000
Stated interest rate		4.75%
Debt maturity date		Oct. 15, 2027