Fair Value of Financial Assets and Liabilities - Fair Value Hierarchy of Financial Assets and Liabilities under U.S. GAAP Guidance (Detail) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Assets:
Marketable securities	$ 8,622	$ 57,623
Loans held for sale	965,639	362,635
Rate lock commitments	8,750	2,923
Forwards	9,687	3,753
Total assets	992,698	426,934
Liabilities:
Accounts payable, accrued expenses and other liabilities—contingent consideration	23,087	23,711
Rate lock commitments	8,980	2,390
Forwards	2,421	657
Total Liabilities	34,488	26,758
Level 1 [Member]
Assets:
Marketable securities	8,622	57,623
Total assets	8,622	57,623
Level 2 [Member]
Assets:
Loans held for sale	965,639	362,635
Total assets	965,639	362,635
Level 3 [Member]
Assets:
Rate lock commitments	8,750	2,923
Forwards	9,687	3,753
Total assets	18,437	6,676
Liabilities:
Accounts payable, accrued expenses and other liabilities—contingent consideration	23,087	23,711
Rate lock commitments	8,980	2,390
Forwards	2,421	657
Total Liabilities	$ 34,488	$ 26,758