Investment Securities - Additional Information on Securities in a Continuous Loss Position (Detail) $ in Thousands	3 Months Ended	12 Months Ended
	Mar. 31, 2018 USD ($) Security	Dec. 31, 2017 USD ($) Security
Schedule of Available-for-sale Securities
Number of debt securities with unrealized losses | Security	635	544
Debt securities with unrealized losses (percentage of amortized cost)	2.87%	1.52%
Number of securities | Security	213	210
Amortized cost basis	$ 1,345,920	$ 1,363,559
Unrealized loss	$ 57,355	$ 36,744
Mortgage-backed securities
Schedule of Available-for-sale Securities
Number of securities | Security	184	181
Amortized cost basis	$ 1,263,477	$ 1,280,739
Unrealized loss	$ 54,057	$ 35,412
Obligations of state and political subdivisions
Schedule of Available-for-sale Securities
Number of securities | Security	28	28
Amortized cost basis	$ 72,443	$ 72,820
Unrealized loss	$ 3,063	$ 1,180
Other
Schedule of Available-for-sale Securities
Number of securities | Security	1	1
Amortized cost basis	$ 10,000	$ 10,000
Unrealized loss	$ 235	$ 152