Investment Securities - Additional Information on Securities in a Continuous Loss Position (Detail) $ in Thousands	6 Months Ended	12 Months Ended
	Jun. 30, 2018 USD ($) Security	Dec. 31, 2017 USD ($) Security
Schedule of Available-for-sale Securities
Number of debt securities with unrealized losses | Security	656	544
Debt securities with unrealized losses (percentage of amortized cost)	3.28%	1.52%
Number of securities | Security	227	210
Amortized cost basis	$ 1,412,036	$ 1,363,559
Unrealized loss	$ 67,235	$ 36,744
Mortgage-backed securities
Schedule of Available-for-sale Securities
Number of securities | Security	193	181
Amortized cost basis	$ 1,301,428	$ 1,280,739
Unrealized loss	$ 62,822	$ 35,412
Obligations of state and political subdivisions
Schedule of Available-for-sale Securities
Number of securities | Security	30	28
Amortized cost basis	$ 76,443	$ 72,820
Unrealized loss	$ 3,115	$ 1,180
Other
Schedule of Available-for-sale Securities
Number of securities | Security	4	1
Amortized cost basis	$ 34,165	$ 10,000
Unrealized loss	$ 1,298	$ 152