Investment Securities - Additional Information on Securities in a Continuous Loss Position (Detail) $ in Thousands	12 Months Ended
	Dec. 31, 2019 USD ($) Security	Dec. 31, 2018 USD ($) Security
Schedule of Available-for-sale Securities
Number of debt securities with unrealized losses | Security	123	488
Debt securities with unrealized losses (percentage of amortized cost)	0.58%	2.38%
Number of securities | Security	69	441
Amortized cost basis	$ 294,631	$ 2,985,374
Unrealized loss	$ 2,382	$ 75,925
Residential agency
Schedule of Available-for-sale Securities
Number of securities | Security	59	302
Amortized cost basis	$ 279,587	$ 2,268,608
Unrealized loss	$ 2,130	$ 57,268
Commercial agency
Schedule of Available-for-sale Securities
Number of securities | Security	6	72
Amortized cost basis	$ 10,453	$ 498,321
Unrealized loss	$ 152	$ 14,756
Obligations of state and political subdivisions
Schedule of Available-for-sale Securities
Number of securities | Security	0	60
Amortized cost basis	$ 0	$ 185,175
Unrealized loss	$ 0	$ 3,208
Other securities
Schedule of Available-for-sale Securities
Number of securities | Security	4	7
Amortized cost basis	$ 4,591	$ 33,270
Unrealized loss	$ 100	$ 693