Accounts Payable and Accrued Expenses - Schedule of Accounts Payable and Accrued Expenses (Detail) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Payables and Accruals [Abstract]
Accounts payable	$ 28,377	$ 34,632
Accrued salaries and bonuses	144,911	225,429
Accrued payroll taxes and benefits	71,379	75,672
Accrued professional liabilities	35,253	37,912
Accrued contingent consideration	26,783	6,259
Accrued interest	5,077	4,495
Other accrued expenses	46,475	53,618
Accounts payable and accrued expenses, total	$ 358,255	$ 438,017