Investment Securities - Amortized Cost, Gross Unrealized Gains and Losses and Fair Value of Investment Securities Held to Maturity (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Schedule of Held-to-Maturity Securities [Line Items]
Amortized Cost	$ 2,173,011	$ 2,336,095
Gross Unrealized Gains	62,326	34,556
Gross Unrealized Losses	(7,725)	(11,931)
Fair Value	2,227,612	2,358,720
U.S. Treasury securities
Schedule of Held-to-Maturity Securities [Line Items]
Amortized Cost	68,126	138,352
Gross Unrealized Gains	7,358	5,761
Gross Unrealized Losses	0	0
Fair Value	75,484	144,113
U.S. government agency securities
Schedule of Held-to-Maturity Securities [Line Items]
Amortized Cost	6,222	7,345
Gross Unrealized Gains	291	58
Gross Unrealized Losses	0	(41)
Fair Value	6,513	7,362
Obligations of states and state agencies
Schedule of Held-to-Maturity Securities [Line Items]
Amortized Cost	262,762	297,454
Gross Unrealized Gains	8,060	7,745
Gross Unrealized Losses	(105)	(529)
Fair Value	270,717	304,670
Municipal bonds
Schedule of Held-to-Maturity Securities [Line Items]
Amortized Cost	207,497	203,251
Gross Unrealized Gains	6,292	5,696
Gross Unrealized Losses	0	(10)
Fair Value	213,789	208,937
Total obligations of states and political subdivisions
Schedule of Held-to-Maturity Securities [Line Items]
Amortized Cost	470,259	500,705
Gross Unrealized Gains	14,352	13,441
Gross Unrealized Losses	(105)	(539)
Fair Value	484,506	513,607
Residential mortgage-backed securities
Schedule of Held-to-Maturity Securities [Line Items]
Amortized Cost	1,550,306	1,620,119
Gross Unrealized Gains	39,603	14,803
Gross Unrealized Losses	(254)	(5,350)
Fair Value	1,589,655	1,629,572
Trust preferred securities
Schedule of Held-to-Maturity Securities [Line Items]
Amortized Cost	37,348	37,324
Gross Unrealized Gains	50	39
Gross Unrealized Losses	(7,365)	(5,981)
Fair Value	30,033	31,382
Corporate and other debt securities
Schedule of Held-to-Maturity Securities [Line Items]
Amortized Cost	40,750	32,250
Gross Unrealized Gains	672	454
Gross Unrealized Losses	(1)	(20)
Fair Value	$ 41,421	$ 32,684

Amount of investment in debt security measured at amortized cost (held-to-maturity).

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
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-Section 45
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Reference 3: http://www.xbrl.org/2003/role/disclosureRef
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Amount of accumulated unrecognized gain on investment in debt security measured at amortized cost (held-to-maturity).

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