Fair Value of Financial Assets and Liabilities - Fair Value Hierarchy of Financial Assets and Liabilities under U.S. GAAP Guidance (Detail) - USD ($) $ in Thousands		Sep. 30, 2019	Dec. 31, 2018
Assets:
Marketable securities		$ 123,417	$ 48,942
Loans held for sale, at fair value		704,357	990,864
Derivative contract, Assets		68,696	92,528
Total		896,470	1,132,334
Liabilities:
Accounts payable, accrued expenses and other liabilities—contingent consideration		40,051	32,552
Derivative contract, Liabilities		27,169	16,678
Total		67,220	49,230
Forward Contracts [Member]
Assets:
Derivative contract, Assets	[1]	42,005	85,796
Liabilities:
Derivative contract, Liabilities		25,971	9,208
Rate Lock Commitments [Member]
Assets:
Derivative contract, Assets		26,691	6,732
Liabilities:
Derivative contract, Liabilities		1,198	7,470
Forward Sale Contracts [Member]
Assets:
Derivative contract, Assets		1,567	8,177
Liabilities:
Derivative contract, Liabilities		25,971	9,208
NASDAQ [Member] | Forward Contracts [Member]
Assets:
Nasdaq Forwards		40,438	77,619
Level 1 [Member]
Assets:
Marketable securities		123,417	48,942
Total		123,417	48,942
Level 2 [Member]
Assets:
Loans held for sale, at fair value		704,357	990,864
Total		704,357	990,864
Level 3 [Member]
Assets:
Total		68,696	92,528
Liabilities:
Accounts payable, accrued expenses and other liabilities—contingent consideration		40,051	32,552
Total		67,220	49,230
Level 3 [Member] | Rate Lock Commitments [Member]
Assets:
Derivative contract, Assets		26,691	6,732
Liabilities:
Derivative contract, Liabilities		1,198	7,470
Level 3 [Member] | Forward Sale Contracts [Member]
Assets:
Derivative contract, Assets		1,567	8,177
Liabilities:
Derivative contract, Liabilities		25,971	9,208
Level 3 [Member] | NASDAQ [Member] | Forward Contracts [Member]
Assets:
Nasdaq Forwards		$ 40,438	$ 77,619

[1] Included in Forwards is $40.4 million and $77.6 million of the Nasdaq Forwards as of September 30, 2019 and December 31, 2018, respectively (See Note 1 — “Organization and Basis of Presentation”). Unrealized gains/(losses) for the change in the fair value of the Nasdaq Forwards for the three months ended September 30, 2019 and 2018 was $(8.2) million and $9.1 million, respectively and $(37.2) million and $6.3 million for the nine months ended September 30, 2019 and 2018, respectively.