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	Sep. 30, 2017	Dec. 31, 2016
Fair Value Disclosures [Abstract]
Loans held for sale, unpaid principal balance	$ 303,953	$ 0
Loans held for sale, fair value	311,186	0
Loans held for sale, fair value over/(under) unpaid principal balance	$ 7,233	$ 0