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	Sep. 30, 2018	Dec. 31, 2017
Mortgage loans held for sale, at fair value
Aggregate Fair Value	$ 42,976	$ 134,916
Aggregate Unpaid Principal	41,848	131,276
Aggregate Fair Value Less Unpaid Principal	1,128	3,640
Mortgage loans held for investment, at fair value
Aggregate Fair Value	7,249	14,953
Aggregate Unpaid Principal	8,666	16,306
Aggregate Fair Value Less Unpaid Principal	$ (1,417)	$ (1,353)