Fair Value of Financial Assets and Liabilities - Changes in Level 3 Assets and Liabilities Measured at Fair Value on Recurring Basis (Detail) - USD ($) $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2020	Dec. 31, 2019
Assets, Unobservable Input Reconciliation:
Opening Balance	$ 72,926	$ 92,528
Total realized and unrealized gains (losses) included in Net income (loss)	47,643	(4,693)
Issuances	0	0
Settlements	(46,424)	(14,909)
Closing Balance	74,145	72,926
Unrealized gains (losses) outstanding, at end of period	74,145	72,926
Liabilities, Unobservable Input Reconciliation:
Opening Balance	70,833	49,229
Total realized and unrealized (gains) losses included in Net income (loss)	30,829	27,948
Issuances	2,221	14,957
Settlements	(29,445)	(21,301)
Closing Balance	74,438	70,833
Unrealized (gains) losses outstanding, at end of period	43,008	27,948
Rate lock commitments
Assets, Unobservable Input Reconciliation:
Opening Balance	32,035	6,732
Total realized and unrealized gains (losses) included in Net income (loss)	45,561	32,035
Issuances	0	0
Settlements	(32,035)	(6,732)
Closing Balance	45,561	32,035
Unrealized gains (losses) outstanding, at end of period	45,561	32,035
Liabilities, Unobservable Input Reconciliation:
Opening Balance	12,124	7,470
Total realized and unrealized (gains) losses included in Net income (loss)	1,133	12,124
Issuances	0	0
Settlements	(12,124)	(7,470)
Closing Balance	1,133	12,124
Unrealized (gains) losses outstanding, at end of period	1,133	12,124
Contingent consideration
Liabilities, Unobservable Input Reconciliation:
Opening Balance	45,172	32,551
Total realized and unrealized (gains) losses included in Net income (loss)	(12,034)	2,287
Issuances	2,221	14,957
Settlements	(3,784)	(4,623)
Closing Balance	31,575	45,172
Unrealized (gains) losses outstanding, at end of period	145	2,287
Forward sale contracts
Assets, Unobservable Input Reconciliation:
Opening Balance	14,389	8,177
Total realized and unrealized gains (losses) included in Net income (loss)	9,625	14,389
Issuances	0	0
Settlements	(14,389)	(8,177)
Closing Balance	9,625	14,389
Unrealized gains (losses) outstanding, at end of period	9,625	14,389
Liabilities, Unobservable Input Reconciliation:
Opening Balance	13,537	9,208
Total realized and unrealized (gains) losses included in Net income (loss)	41,730	13,537
Issuances	0	0
Settlements	(13,537)	(9,208)
Closing Balance	41,730	13,537
Unrealized (gains) losses outstanding, at end of period	41,730	13,537
Nasdaq Forwards
Assets, Unobservable Input Reconciliation:
Opening Balance	26,502	77,619
Total realized and unrealized gains (losses) included in Net income (loss)	(7,543)	(51,117)
Issuances	0	0
Settlements	0	0
Closing Balance	18,959	26,502
Unrealized gains (losses) outstanding, at end of period	$ 18,959	$ 26,502