Investment Securities - Amortized Cost, Held to Maturity (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Schedule of Held-to-maturity Securities
Amortized Cost	$ 2,072,363	$ 1,842,691
Gross Unrealized Gains	9,789	25,419
Gross Unrealized Losses	(65,798)	(30,490)
Fair Value	2,016,354	1,837,620
U.S. Treasury securities
Schedule of Held-to-maturity Securities
Amortized Cost	138,557	138,676
Gross Unrealized Gains	1,853	6,581
Gross Unrealized Losses	(141)	0
Fair Value	140,269	145,257
U.S. government agency securities
Schedule of Held-to-maturity Securities
Amortized Cost	8,846	9,859
Gross Unrealized Gains	0	122
Gross Unrealized Losses	(305)	0
Fair Value	8,541	9,981
Obligations of states and state agencies
Schedule of Held-to-maturity Securities
Amortized Cost	347,775	244,272
Gross Unrealized Gains	3,717	7,083
Gross Unrealized Losses	(6,970)	(1,653)
Fair Value	344,522	249,702
Municipal bonds
Schedule of Held-to-maturity Securities
Amortized Cost	248,585	221,606
Gross Unrealized Gains	2,561	6,199
Gross Unrealized Losses	(2,245)	(28)
Fair Value	248,901	227,777
Obligations of states and political subdivisions
Schedule of Held-to-maturity Securities
Amortized Cost	596,360	465,878
Gross Unrealized Gains	6,278	13,282
Gross Unrealized Losses	(9,215)	(1,681)
Fair Value	593,423	477,479
Residential mortgage-backed securities
Schedule of Held-to-maturity Securities
Amortized Cost	1,259,772	1,131,945
Gross Unrealized Gains	1,573	4,842
Gross Unrealized Losses	(49,589)	(18,743)
Fair Value	1,211,756	1,118,044
Trust preferred securities
Schedule of Held-to-maturity Securities
Amortized Cost	37,328	49,824
Gross Unrealized Gains	38	60
Gross Unrealized Losses	(6,213)	(9,796)
Fair Value	31,153	40,088
Corporate and other debt securities
Schedule of Held-to-maturity Securities
Amortized Cost	31,500	46,509
Gross Unrealized Gains	47	532
Gross Unrealized Losses	(335)	(270)
Fair Value	$ 31,212	$ 46,771

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

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-Publisher FASB
-Name Accounting Standards Codification
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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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