Fair Value Measurements - Amount of Net Gains (Losses) from Fair Value Changes (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Fair Value Disclosures [Abstract]
Net Gains (Losses) Resulting From Changes in Fair Value - Mortgage loans held for sale, at fair value	$ (1,041)	$ 251	$ (2,513)	$ 1,121
Net Gains (Losses) Resulting From Changes in Fair Value - Mortgage loans held for investment, at fair value	$ (296)	$ (62)	$ (1,217)	$ (127)