Fair Value of Financial Assets and Liabilities - Additional Information (Detail) - USD ($)	9 Months Ended	12 Months Ended
	Sep. 30, 2019	Dec. 31, 2018	Jun. 30, 2019	Sep. 30, 2018	Jun. 30, 2018	Dec. 31, 2017
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Fair value assets transfers from level 1 to level 2	$ 0	$ 0
Fair value assets transfers from level 2 to level 1	0	0
Fair value liabilities transfers from level 1 to level 2	0	0
Fair value liabilities transfers from level 2 to level 1	0	0
Fair value assets transfers Into level 3	0	0
Fair value assets transfers out of level 3	0	0
Fair value liabilities transfers Into level 3	0	0
Fair value liabilities transfers out of level 3	0	0
Present value of expected payments related to contingent consideration	67,220,000	49,230,000	$ 68,401,000	$ 43,731,000	$ 36,159,000	$ 26,758,000
Other Assets [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Alternative investment	102,400,000	53,500,000
Other Assets [Member] | Fair Value, Measurements, Nonrecurring [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Alternative investment	102,400,000	53,500,000
Contingent Consideration [Member] | Accounts Payable, Accrued and Other Liabilities [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Present value of expected payments related to contingent consideration	40,051,000	32,552,000	$ 38,712,000	$ 31,605,000	$ 21,573,000	$ 23,711,000
Contingent Consideration [Member] | Level 3 [Member] | Accounts Payable, Accrued and Other Liabilities [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Present value of expected payments related to contingent consideration	40,100,000	32,600,000
Undiscounted value of payments, assuming that all contingencies are met	$ 45,000,000.0	$ 39,600,000