Investment Securities - Additional Information on Securities in a Continuous Loss Position (Detail) $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2019 USD ($) Security	Dec. 31, 2018 USD ($) Security
Schedule of Available-for-sale Securities
Number of debt securities with unrealized losses | Security	107	488
Debt securities with unrealized losses (percentage of amortized cost)	0.55%	2.38%
Number of securities | Security	76	441
Amortized cost basis	$ 327,233	$ 2,985,374
Unrealized loss	$ 2,490	$ 75,925
Mortgage-backed securities: Residential agency
Schedule of Available-for-sale Securities
Number of securities | Security	66	302
Amortized cost basis	$ 311,242	$ 2,268,608
Unrealized loss	$ 2,295	$ 57,268
Mortgage-backed securities: Commercial agency
Schedule of Available-for-sale Securities
Number of securities | Security	6	72
Amortized cost basis	$ 11,090	$ 498,321
Unrealized loss	$ 96	$ 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	$ 4,901	$ 33,270
Unrealized loss	$ 99	$ 693