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	Mar. 31, 2019	Dec. 31, 2018
Mortgage loans held for sale, at fair value
Aggregate Fair Value	$ 128,451	$ 107,734
Aggregate Unpaid Principal	124,410	104,345
Aggregate Fair Value Less Unpaid Principal	4,041	3,389
Mortgage loans held for investment, at fair value
Aggregate Fair Value	550	3,143
Aggregate Unpaid Principal	599	3,595
Aggregate Fair Value Less Unpaid Principal	$ (49)	$ (452)