Fair Value Measurements - Summary of Difference Between the Aggregate Fair Value and the Aggregate Unpaid Principal Balance for Mortgage Loans Held for Sale (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017
Fair Value Disclosures [Abstract]
Fair value of acquired loans at acquisition	$ 17,000		$ 15,000
Net gains (losses) resulting from change in fair value of loans held-for-investment	(749)	$ (409)
Mortgages Held For Sale at Fair Value
Aggregate fair value	110,348		134,916
Aggregate unpaid principal	107,849		131,276
Aggregate fair value less unpaid principal	2,499		$ 3,640
Net gains (losses) resulting from change in fair value of loans held-for-sale	$ (1,100)	$ 839