Fair Value Measurements - Summary of Difference Between Aggregate Fair Value and Remaining Contractual Principal Outstanding for Financial Instruments for which Fair Value Option has been Elected (Detail) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Fair Value Disclosures [Abstract]
Loans held for sale, unpaid principal balance	$ 275,693	$ 257,674
Loans held for sale, fair value	282,439	263,308
Loans held for sale, fair value over/(under) unpaid principal balance	$ 6,746	$ 5,634