Investment Securities - Amortized Cost, Securities Available for Sale (Details) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	$ 1,677,715	$ 1,796,404
Gross Unrealized Gains	10,677	1,995
Gross Unrealized Losses	(9,042)	(48,855)
Fair Value	1,679,350	1,749,544
U.S. Treasury securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	50,963	50,975
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(222)	(1,669)
Fair Value	50,741	49,306
U.S. government agency securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	32,525	36,844
Gross Unrealized Gains	521	71
Gross Unrealized Losses	(18)	(638)
Fair Value	33,028	36,277
Obligations of states and state agencies
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	87,473	100,777
Gross Unrealized Gains	465	18
Gross Unrealized Losses	(222)	(3,682)
Fair Value	87,716	97,113
Municipal bonds
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	98,305	101,207
Gross Unrealized Gains	899	209
Gross Unrealized Losses	(223)	(1,437)
Fair Value	98,981	99,979
Obligations of states and political subdivisions
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	185,778	201,984
Gross Unrealized Gains	1,364	227
Gross Unrealized Losses	(445)	(5,119)
Fair Value	186,697	197,092
Residential mortgage-backed securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	1,373,033	1,469,059
Gross Unrealized Gains	8,303	1,484
Gross Unrealized Losses	(8,357)	(40,761)
Fair Value	1,372,979	1,429,782
Corporate and other debt securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	35,416	37,542
Gross Unrealized Gains	489	213
Gross Unrealized Losses	0	(668)
Fair Value	$ 35,905	$ 37,087

Amount, before tax, of unrealized gain in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
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Amount, before tax, of unrealized loss in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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Amortized cost of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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-Publisher FASB
-Name Accounting Standards Codification
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Amount of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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-Publisher FASB
-Name Accounting Standards Codification
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