Accounts Payable and Accrued Expenses - Schedule of Accounts Payable and Accrued Expenses (Detail) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Payables and Accruals [Abstract]
Accounts payable	$ 32,149	$ 31,059
Accrued salaries and bonuses	105,565	242,888
Accrued payroll taxes and benefits	52,385	78,415
Accrued professional liabilities	41,198	34,931
Accrued contingent consideration	15,069	18,760
Accrued interest	26,516	9,477
Other accrued expenses	40,075	33,037
Accounts payable and accrued expenses, total	$ 312,957	$ 448,567