Fair Value of Financial Assets and Liabilities - Quantitative Information about Level 3 Fair Value Measurements (Detail) $ in Thousands	Sep. 30, 2019 USD ($)	Jun. 30, 2019 USD ($)	Dec. 31, 2018 USD ($)	Sep. 30, 2018 USD ($)	Jun. 30, 2018 USD ($)	Dec. 31, 2017 USD ($)
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Assets	$ 68,696	$ 73,448	$ 92,528	$ 110,763	$ 49,975	$ 6,676
Liabilities	67,220	68,401	49,230	43,731	36,159	26,758
Contingent Consideration [Member] | Accounts Payable, Accrued and Other Liabilities [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Liabilities	$ 40,051	38,712	$ 32,552	31,605	21,573	23,711
Contingent Consideration [Member] | Accounts Payable, Accrued and Other Liabilities [Member] | Minimum [Member] | Measurement Input, Discount Rate [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Unobservable Inputs	0.003		0.003
Contingent Consideration [Member] | Accounts Payable, Accrued and Other Liabilities [Member] | Minimum [Member] | Measurement Input, Probability of Meeting Earnout and Contingencies [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Unobservable Inputs	0.99		0.99
Contingent Consideration [Member] | Accounts Payable, Accrued and Other Liabilities [Member] | Maximum [Member] | Measurement Input, Discount Rate [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Unobservable Inputs	0.104		0.104
Contingent Consideration [Member] | Accounts Payable, Accrued and Other Liabilities [Member] | Maximum [Member] | Measurement Input, Probability of Meeting Earnout and Contingencies [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Unobservable Inputs	1.00		1.00
Contingent Consideration [Member] | Accounts Payable, Accrued and Other Liabilities [Member] | Weighted Average [Member] | Measurement Input, Discount Rate [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Unobservable Inputs	0.080		0.080
Contingent Consideration [Member] | Accounts Payable, Accrued and Other Liabilities [Member] | Weighted Average [Member] | Measurement Input, Probability of Meeting Earnout and Contingencies [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Unobservable Inputs	0.995		0.996
Forward Contracts [Member] | NASDAQ [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Assets	$ 40,438	48,652	$ 77,619	84,029	19,085
Forward Contracts [Member] | NASDAQ [Member] | Minimum [Member] | Volatility [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Unobservable Inputs	0.2804		0.237
Forward Contracts [Member] | NASDAQ [Member] | Maximum [Member] | Volatility [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Unobservable Inputs	0.3562		0.348
Forward Contracts [Member] | NASDAQ [Member] | Weighted Average [Member] | Volatility [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Unobservable Inputs	0.329		0.302
Forward Sale Contracts [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Assets	$ 1,567	2,845	$ 8,177	20,515	16,442	3,753
Liabilities	25,971	27,261	9,208	37	641	657
Rate Lock Commitments [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Assets	26,691	21,951	6,732	6,219	14,448	2,923
Liabilities	$ 1,198	$ 2,428	$ 7,470	$ 12,089	$ 13,945	$ 2,390