Investment Securities Available-for-Sale - Amortized cost and estimated fair value of securities available-for-sale (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019
Amortized Cost	$ 956,803	$ 839,192
Gross Unrealized Gains	22,363	7,206
Gross Unrealized Losses	(1,440)	(3,035)
Allowance for Credit Losses	(156)	0
Investment securities	977,570	843,363
U. S. agency securities
Amortized Cost	130,313	180,228
Gross Unrealized Gains	1,638	621
Gross Unrealized Losses	(665)	(1,055)
Investment securities	131,286	179,794
Residential mortgage backed securities
Amortized Cost	702,655	541,490
Gross Unrealized Gains	14,578	4,337
Gross Unrealized Losses	(590)	(1,975)
Investment securities	716,643	543,852
Municipal bonds
Amortized Cost	90,292	71,902
Gross Unrealized Gains	4,551	2,034
Gross Unrealized Losses	(114)	(5)
Allowance for Credit Losses	(16)
Investment securities	94,713	73,931
Corporate bonds
Amortized Cost	33,345	10,530
Gross Unrealized Gains	1,596	203
Gross Unrealized Losses	(71)
Allowance for Credit Losses	(140)
Investment securities	34,730	10,733
U.S. Treasury
Amortized Cost		34,844
Gross Unrealized Gains		11
Investment securities		34,855
Other equity investments
Amortized Cost	198	198
Investment securities	$ 198	$ 198

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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