Investment Securities - Additional Information on Securities in a Continuous Loss Position (Detail) $ in Thousands	3 Months Ended	12 Months Ended
	Mar. 31, 2019 USD ($) Security	Dec. 31, 2018 USD ($) Security
Schedule of Available-for-sale Securities
Number of debt securities with unrealized losses | Security	389	488
Debt securities with unrealized losses (percentage of amortized cost)	1.45%	2.38%
Number of securities | Security	379	441
Amortized cost basis	$ 2,630,889	$ 2,985,374
Unrealized loss	$ 38,729	$ 75,925
Mortgage-backed securities: Residential agency
Schedule of Available-for-sale Securities
Number of securities | Security	289	302
Amortized cost basis	$ 2,129,466	$ 2,268,608
Unrealized loss	$ 30,434	$ 57,268
Mortgage-backed securities: Commercial agency
Schedule of Available-for-sale Securities
Number of securities | Security	65	72
Amortized cost basis	$ 442,211	$ 498,321
Unrealized loss	$ 7,711	$ 14,756
Obligations of state and political subdivisions
Schedule of Available-for-sale Securities
Number of securities | Security	17	60
Amortized cost basis	$ 25,159	$ 185,175
Unrealized loss	$ 74	$ 3,208
Other securities
Schedule of Available-for-sale Securities
Number of securities | Security	8	7
Amortized cost basis	$ 34,053	$ 33,270
Unrealized loss	$ 510	$ 693