Investment Securities - Amortized Cost, Held to Maturity (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Schedule of Held-to-maturity Securities
Amortized Cost	$ 2,074,399	$ 2,068,246
Gross Unrealized Gains	22,175	14,436
Gross Unrealized Losses	(29,604)	(47,739)
Fair Value	2,066,970	2,034,943
U.S. Treasury securities
Schedule of Held-to-maturity Securities
Amortized Cost	138,478	138,517
Gross Unrealized Gains	4,421	3,532
Gross Unrealized Losses	0	0
Fair Value	142,899	142,049
U.S. government agency securities
Schedule of Held-to-maturity Securities
Amortized Cost	8,236	8,721
Gross Unrealized Gains	16	55
Gross Unrealized Losses	(183)	(135)
Fair Value	8,069	8,641
Obligations of states and state agencies
Schedule of Held-to-maturity Securities
Amortized Cost	315,954	341,702
Gross Unrealized Gains	5,994	4,332
Gross Unrealized Losses	(2,086)	(5,735)
Fair Value	319,862	340,299
Municipal bonds
Schedule of Held-to-maturity Securities
Amortized Cost	235,547	243,954
Gross Unrealized Gains	4,562	3,141
Gross Unrealized Losses	(133)	(1,361)
Fair Value	239,976	245,734
Obligations of states and political subdivisions
Schedule of Held-to-maturity Securities
Amortized Cost	551,501	585,656
Gross Unrealized Gains	10,556	7,473
Gross Unrealized Losses	(2,219)	(7,096)
Fair Value	559,838	586,033
Residential mortgage-backed securities
Schedule of Held-to-maturity Securities
Amortized Cost	1,306,625	1,266,770
Gross Unrealized Gains	6,970	3,203
Gross Unrealized Losses	(21,231)	(34,368)
Fair Value	1,292,364	1,235,605
Trust preferred securities
Schedule of Held-to-maturity Securities
Amortized Cost	37,309	37,332
Gross Unrealized Gains	68	77
Gross Unrealized Losses	(5,776)	(5,923)
Fair Value	31,601	31,486
Corporate and other debt securities
Schedule of Held-to-maturity Securities
Amortized Cost	32,250	31,250
Gross Unrealized Gains	144	96
Gross Unrealized Losses	(195)	(217)
Fair Value	$ 32,199	$ 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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