Accounts Payable and Accrued Expenses - Schedule of Accounts Payable and Accrued Expenses (Detail) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Payables and Accruals [Abstract]
Accounts payable	$ 31,059	$ 34,632
Accrued salaries and bonuses	249,834	225,429
Accrued payroll taxes and benefits	80,369	75,672
Accrued professional liabilities	34,931	37,912
Accrued contingent consideration	18,760	6,259
Accrued interest	9,477	4,495
Other accrued expenses	44,912	53,618
Accounts payable and accrued expenses, total	$ 469,342	$ 438,017