Investment Securities - Additional Information on Securities in a Continuous Loss Position (Detail) $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2018 USD ($) Security	Dec. 31, 2017 USD ($) Security
Schedule of Available-for-sale Securities
Number of debt securities with unrealized losses | Security	719	544
Debt securities with unrealized losses (percentage of amortized cost)	3.72%	1.52%
Number of securities | Security	464	210
Amortized cost basis	$ 3,267,464	$ 1,363,559
Unrealized loss	$ 148,043	$ 36,744
Mortgage-backed securities: Residential agency
Schedule of Available-for-sale Securities
Number of securities | Security	336	153
Amortized cost basis	$ 2,618,769	$ 1,110,834
Unrealized loss	$ 118,495	$ 29,977
Mortgage-backed securities: Commercial agency
Schedule of Available-for-sale Securities
Number of securities | Security	65	28
Amortized cost basis	$ 440,151	$ 169,905
Unrealized loss	$ 21,856	$ 5,435
Obligations of state and political subdivisions
Schedule of Available-for-sale Securities
Number of securities | Security	53	28
Amortized cost basis	$ 137,286	$ 72,820
Unrealized loss	$ 5,565	$ 1,180
U.S. Government-sponsored enterprise obligations
Schedule of Available-for-sale Securities
Number of securities | Security	3	1
Amortized cost basis	$ 39,999	$ 10,000
Unrealized loss	$ 740	$ 152
Other securities
Schedule of Available-for-sale Securities
Number of securities | Security	7	0
Amortized cost basis	$ 31,259	$ 0
Unrealized loss	$ 1,387	$ 0