Investment Securities - Temporarily Impaired Securities (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017	Sep. 30, 2017
Debt Securities, Available-for-sale [Line Items]
Fair Value (Estimated) (Less than 12 months)	$ 267,597	$ 148,611	$ 194,691
Unrealized Losses (Less than 12 months)	(7,835)	(2,100)	(3,018)
Fair Value (Estimated) (12 months or more)	189,876	144,394	60,608
Unrealized Losses (12 months or more)	(12,273)	(3,843)	(1,760)
Fair Value (Estimated)	457,473	293,005	255,299
Unrealized Losses	(20,108)	(5,943)	(4,778)
U.S. Government-sponsored agencies
Debt Securities, Available-for-sale [Line Items]
Fair Value (Estimated) (Less than 12 months)	7,753	7,161	7,958
Unrealized Losses (Less than 12 months)	(247)	(94)	(197)
Fair Value (Estimated) (12 months or more)	7,614	3,814	0
Unrealized Losses (12 months or more)	(541)	(86)	0
Fair Value (Estimated)	15,367	10,975	7,958
Unrealized Losses	(788)	(180)	(197)
Mortgage-backed securities
Debt Securities, Available-for-sale [Line Items]
Fair Value (Estimated) (Less than 12 months)	154,611	132,025	147,326
Unrealized Losses (Less than 12 months)	(4,333)	(1,857)	(1,635)
Fair Value (Estimated) (12 months or more)	143,771	101,707	48,015
Unrealized Losses (12 months or more)	(7,427)	(2,693)	(1,106)
Fair Value (Estimated)	298,382	233,732	195,341
Unrealized Losses	(11,760)	(4,550)	(2,741)
State and political subdivisions
Debt Securities, Available-for-sale [Line Items]
Fair Value (Estimated) (Less than 12 months)	105,233	9,425	39,407
Unrealized Losses (Less than 12 months)	(3,255)	(149)	(1,186)
Fair Value (Estimated) (12 months or more)	38,491	38,864	12,583
Unrealized Losses (12 months or more)	(4,305)	(1,061)	(652)
Fair Value (Estimated)	143,724	48,289	51,990
Unrealized Losses	$ (7,560)	(1,210)	(1,838)
Other equity securities
Debt Securities, Available-for-sale [Line Items]
Fair Value (Estimated) (Less than 12 months)		0	0
Unrealized Losses (Less than 12 months)		0	0
Fair Value (Estimated) (12 months or more)		9	10
Unrealized Losses (12 months or more)		(3)	(2)
Fair Value (Estimated)		9	10
Unrealized Losses		$ (3)	$ (2)

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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