Investment Securities Held-to-Maturity - Summary of Held-to-Maturity Securities (Detail) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	$ 2,509,088	$ 2,351,120
Gross Unrealized Gains	71,913	21,429
Gross Unrealized Losses	1,898	11,245
Estimated Fair Value	2,579,103	2,361,304
U.S. Government Sponsored Enterprises [Member]
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	244,220	98,867
Gross Unrealized Gains	389	527
Gross Unrealized Losses	866	96
Estimated Fair Value	243,743	99,298
SBA Backed Securities [Member]
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	37,783	44,379
Gross Unrealized Gains	2,002	182
Gross Unrealized Losses	0	303
Estimated Fair Value	39,785	44,258
Mortgage-Backed Securities [Member]
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	2,227,085	2,207,874
Gross Unrealized Gains	69,522	20,720
Gross Unrealized Losses	1,032	10,846
Estimated Fair Value	$ 2,295,575	$ 2,217,748