Investment Securities - Amortized Cost, Held to Maturity (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Schedule of Held-to-maturity Securities
Amortized Cost	$ 2,093,757	$ 2,068,246
Gross Unrealized Gains	38,404	14,436
Gross Unrealized Losses	(10,958)	(47,739)
Fair Value	2,121,203	2,034,943
U.S. Treasury securities
Schedule of Held-to-maturity Securities
Amortized Cost	138,393	138,517
Gross Unrealized Gains	6,479	3,532
Gross Unrealized Losses	0	0
Fair Value	144,872	142,049
U.S. government agency securities
Schedule of Held-to-maturity Securities
Amortized Cost	7,462	8,721
Gross Unrealized Gains	93	55
Gross Unrealized Losses	(2)	(135)
Fair Value	7,553	8,641
Obligations of states and state agencies
Schedule of Held-to-maturity Securities
Amortized Cost	305,129	341,702
Gross Unrealized Gains	7,885	4,332
Gross Unrealized Losses	(585)	(5,735)
Fair Value	312,429	340,299
Municipal bonds
Schedule of Held-to-maturity Securities
Amortized Cost	212,301	243,954
Gross Unrealized Gains	5,777	3,141
Gross Unrealized Losses	(25)	(1,361)
Fair Value	218,053	245,734
Obligations of states and political subdivisions
Schedule of Held-to-maturity Securities
Amortized Cost	517,430	585,656
Gross Unrealized Gains	13,662	7,473
Gross Unrealized Losses	(610)	(7,096)
Fair Value	530,482	586,033
Residential mortgage-backed securities
Schedule of Held-to-maturity Securities
Amortized Cost	1,360,903	1,266,770
Gross Unrealized Gains	17,687	3,203
Gross Unrealized Losses	(3,765)	(34,368)
Fair Value	1,374,825	1,235,605
Trust preferred securities
Schedule of Held-to-maturity Securities
Amortized Cost	37,319	37,332
Gross Unrealized Gains	53	77
Gross Unrealized Losses	(6,475)	(5,923)
Fair Value	30,897	31,486
Corporate and other debt securities
Schedule of Held-to-maturity Securities
Amortized Cost	32,250	31,250
Gross Unrealized Gains	430	96
Gross Unrealized Losses	(106)	(217)
Fair Value	$ 32,574	$ 31,129

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

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

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

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