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, 2020	Dec. 31, 2019
Mortgage loans held for sale, at fair value
Aggregate Fair Value	$ 207,845	$ 213,357
Aggregate Unpaid Principal	201,259	207,481
Aggregate Fair Value Less Unpaid Principal	6,586	5,876
Mortgage loans held for investment, at fair value
Aggregate Fair Value	2,779	2,792
Aggregate Unpaid Principal	2,804	2,922
Aggregate Fair Value Less Unpaid Principal	$ (25)	$ (130)