Investment Securities - Additional Information on Securities in a Continuous Loss Position (Detail) $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2017 USD ($) Security	Dec. 31, 2016 USD ($) Security
Schedule of Available-for-sale Securities
Number of debt securities with unrealized losses | Security	439	397
Debt securities with unrealized losses (percentage of amortized cost)	1.02%	1.79%
Number of securities | Security	121	31
Amortized cost basis	$ 579,699	$ 116,886
Unrealized loss	$ 13,168	$ 3,164
Mortgage-backed securities
Schedule of Available-for-sale Securities
Number of securities | Security	91	28
Amortized cost basis	$ 505,161	$ 112,983
Unrealized loss	$ 11,812	$ 3,111
Obligations of state and political subdivisions
Schedule of Available-for-sale Securities
Number of securities | Security	28	0
Amortized cost basis	$ 73,197	$ 0
Unrealized loss	$ 1,339	$ 0
Other
Schedule of Available-for-sale Securities
Number of securities | Security	2	3
Amortized cost basis	$ 1,341	$ 3,903
Unrealized loss	$ 17	$ 53