Investment Securities - Temporarily Impaired Securities (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018	Mar. 31, 2018
Debt Securities, Available-for-sale [Line Items]
Fair Value (Estimated) (Less than 12 months)	$ 28,783	$ 152,859	$ 254,875
Unrealized Losses (Less than 12 months)	(147)	(2,845)	(6,174)
Fair Value (Estimated) (12 months or more)	267,315	240,871	141,888
Unrealized Losses (12 months or more)	(5,448)	(10,271)	(6,686)
Fair Value (Estimated)	296,098	393,730	396,763
Unrealized Losses	(5,595)	(13,116)	(12,860)
U.S. Government-sponsored agencies
Debt Securities, Available-for-sale [Line Items]
Fair Value (Estimated) (Less than 12 months)	0	0	6,946
Unrealized Losses (Less than 12 months)	0	0	(309)
Fair Value (Estimated) (12 months or more)	10,875	10,683	3,694
Unrealized Losses (12 months or more)	(280)	(472)	(206)
Fair Value (Estimated)	10,875	10,683	10,640
Unrealized Losses	(280)	(472)	(515)
Mortgage-backed securities
Debt Securities, Available-for-sale [Line Items]
Fair Value (Estimated) (Less than 12 months)	24,525	76,050	183,335
Unrealized Losses (Less than 12 months)	(138)	(1,061)	(4,332)
Fair Value (Estimated) (12 months or more)	198,272	185,136	101,012
Unrealized Losses (12 months or more)	(3,248)	(5,926)	(3,755)
Fair Value (Estimated)	222,797	261,186	284,347
Unrealized Losses	(3,386)	(6,987)	(8,087)
State and political subdivisions
Debt Securities, Available-for-sale [Line Items]
Fair Value (Estimated) (Less than 12 months)	1,264	76,809	64,594
Unrealized Losses (Less than 12 months)	(3)	(1,784)	(1,533)
Fair Value (Estimated) (12 months or more)	58,168	45,052	37,182
Unrealized Losses (12 months or more)	(1,920)	(3,873)	(2,725)
Fair Value (Estimated)	59,432	121,861	101,776
Unrealized Losses	(1,923)	$ (5,657)	$ (4,258)
Corporate securities
Debt Securities, Available-for-sale [Line Items]
Fair Value (Estimated) (Less than 12 months)	2,994
Unrealized Losses (Less than 12 months)	(6)
Fair Value (Estimated) (12 months or more)	0
Unrealized Losses (12 months or more)	0
Fair Value (Estimated)	2,994
Unrealized Losses	$ (6)

Amount of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), in continuous unrealized loss position for more than 12 months.

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 7
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