Fair Value Measurements - Summary of Difference Between the Aggregate Fair Value and the Aggregate Unpaid Principal Balance for Mortgage Loans Held for Sale and Mortgage Loans Held for Investment (Detail) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Mortgages Held For Sale at Fair Value
Aggregate fair value	$ 78,843	$ 134,916
Aggregate unpaid principal	76,675	131,276
Aggregate fair value less unpaid principal	2,168	3,640
Mortgages Held For Investment at Fair Value [Abstract]
Loans Receivable, Fair Value Disclosure	18,274	14,953
Mortgage Loans Held For Investment Unpaid Principal Balance	20,548	16,306
Mortgages Held For Investment Fair Value In Excess Of Unpaid Principal Balance	$ (2,274)	$ (1,353)