Investment Securities - Amortized Cost, Gross Unrealized Gains and Losses and Fair Value of Debt Securities Held to Maturity (Details) - USD ($) $ in Thousands	Jun. 30, 2021	Dec. 31, 2020
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	$ 2,533,812	$ 2,173,011
Gross Unrealized Gains	40,432	62,326
Gross Unrealized Losses	(17,725)	(7,725)
Fair Value	2,556,519	2,227,612
U.S. Treasury securities
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	67,874	68,126
Gross Unrealized Gains	5,701	7,358
Gross Unrealized Losses	0	0
Fair Value	73,575	75,484
U.S. government agency securities
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	5,548	6,222
Gross Unrealized Gains	265	291
Gross Unrealized Losses	0	0
Fair Value	5,813	6,513
Obligations of states and state agencies
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	243,374	262,762
Gross Unrealized Gains	5,505	8,060
Gross Unrealized Losses	(43)	(105)
Fair Value	248,836	270,717
Municipal bonds
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	201,006	207,497
Gross Unrealized Gains	4,894	6,292
Gross Unrealized Losses	(18)	0
Fair Value	205,882	213,789
Obligations of states and political subdivisions
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	444,380	470,259
Gross Unrealized Gains	10,399	14,352
Gross Unrealized Losses	(61)	(105)
Fair Value	454,718	484,506
Residential mortgage-backed securities
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	1,926,900	1,550,306
Gross Unrealized Gains	23,376	39,603
Gross Unrealized Losses	(11,668)	(254)
Fair Value	1,938,608	1,589,655
Trust preferred securities
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	37,360	37,348
Gross Unrealized Gains	51	50
Gross Unrealized Losses	(5,986)	(7,365)
Fair Value	31,425	30,033
Corporate and other debt securities
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	51,750	40,750
Gross Unrealized Gains	640	672
Gross Unrealized Losses	(10)	(1)
Fair Value	$ 52,380	$ 41,421

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

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