Investment Securities - Amortized Cost, Gross Unrealized Gains and Losses and Fair Value of Debt Securities Held to Maturity (Details) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	$ 2,584,403	$ 2,173,011
Gross Unrealized Gains	35,864	62,326
Gross Unrealized Losses	(23,956)	(7,725)
Fair Value	2,596,311	2,227,612
U.S. Treasury securities
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	67,716	68,126
Gross Unrealized Gains	5,182	7,358
Gross Unrealized Losses	0	0
Fair Value	72,898	75,484
U.S. government agency securities
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	4,817	6,222
Gross Unrealized Gains	198	291
Gross Unrealized Losses	0	0
Fair Value	5,015	6,513
Obligations of states and state agencies
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	161,929	262,762
Gross Unrealized Gains	3,866	8,060
Gross Unrealized Losses	(293)	(105)
Fair Value	165,502	270,717
Municipal bonds
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	191,477	207,497
Gross Unrealized Gains	4,066	6,292
Gross Unrealized Losses	(142)	0
Fair Value	195,401	213,789
Obligations of states and political subdivisions
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	353,406	470,259
Gross Unrealized Gains	7,932	14,352
Gross Unrealized Losses	(435)	(105)
Fair Value	360,903	484,506
Residential mortgage-backed securities
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	2,072,700	1,550,306
Gross Unrealized Gains	21,898	39,603
Gross Unrealized Losses	(18,001)	(254)
Fair Value	2,076,597	1,589,655
Trust preferred securities
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	37,014	37,348
Gross Unrealized Gains	4	50
Gross Unrealized Losses	(5,511)	(7,365)
Fair Value	31,507	30,033
Corporate and other debt securities
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	48,750	40,750
Gross Unrealized Gains	650	672
Gross Unrealized Losses	(9)	(1)
Fair Value	$ 49,391	$ 41,421

Amount, before allowance for credit loss, of investment in debt security measured at amortized cost (held-to-maturity).

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