Investment Securities - Additional Information on Securities in a Continuous Loss Position (Detail) $ in Thousands	6 Months Ended	12 Months Ended
	Jun. 30, 2019 USD ($) Security	Dec. 31, 2018 USD ($) Security
Schedule of Available-for-sale Securities
Number of debt securities with unrealized losses | Security	189	488
Debt securities with unrealized losses (percentage of amortized cost)	0.67%	2.38%
Number of securities | Security	189	441
Amortized cost basis	$ 1,178,504	$ 2,985,374
Unrealized loss	$ 7,905	$ 75,925
Mortgage-backed securities: Residential agency
Schedule of Available-for-sale Securities
Number of securities | Security	159	302
Amortized cost basis	$ 1,049,840	$ 2,268,608
Unrealized loss	$ 6,706	$ 57,268
Mortgage-backed securities: Commercial agency
Schedule of Available-for-sale Securities
Number of securities | Security	26	72
Amortized cost basis	$ 123,557	$ 498,321
Unrealized loss	$ 1,107	$ 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	$ 5,107	$ 33,270
Unrealized loss	$ 92	$ 693