
	June 30, 2019
	Amortized Cost Basis		Gross Unrealized Holding Gains		Gross Unrealized Holding Losses			Estimated Fair Value

U.S. Treasury securities	$	9,984	$	36	$	—		$	10,020
Obligations of states and political subdivisions		1,103,682		53,593		(61	)		1,157,214
Corporate bonds and other		4,643		51		—			4,694
Residential mortgage-backed securities		1,579,544		19,732		(3,101	)		1,596,175
Commercial mortgage-backed securities		484,919		6,929		(459	)		491,389

Total securities available-for-sale	$	3,182,772	$	80,341	$	(3,621	)	$	3,259,492


	December 31, 2019
	Amortized Cost Basis		Gross Unrealized Holding Gains		Gross Unrealized Holding Losses			Estimated Fair Value

U.S. Treasury securities	$	9,997	$	22	$	—		$	10,019
Obligations of states and political subdivisions		1,231,619		57,764		(400	)		1,288,983
Corporate bonds and other		4,643		65		—			4,708
Residential mortgage-backed securities		1,586,872		23,139		(1,148	)		1,608,863
Commercial mortgage-backed securities		494,674		6,356		(286	)		500,744

Total securities available-for-sale	$	3,327,805	$	87,346	$	(1,834	)	$	3,413,317

The Company invests in mortgage-backed securities that have expected maturities that differ from their contractual maturities. These differences arise because borrowers may have the right to call or prepay obligations with or without a prepayment penalty. These securities include collateralized mortgage obligations (CMOs) and other asset backed securities. The expected maturities of these securities at June 30, 2020 were computed by using scheduled amortization of balances and historical prepayment rates. At June 30, 2020 and 2019, and December 31, 2019, the Company did not hold CMOs that entail higher risks than standard mortgage-backed securities.

The amortized cost and estimated fair value of

available-for-sale

securities at June 30, 2020 by contractual and expected maturity, are shown below (in thousands):


	Amortized Cost Basis		Estimated Fair Value
Due within one year	$	128,535	$	129,521
Due after one year through five years		625,856		669,229
Due after five years through ten years		1,079,767		1,148,866
Due after ten years		16,895		17,354
Mortgage-backed securities		2,076,201		2,153,893

Total	$	3,927,254	$	4,118,863

The following tables disclose the Company’s investment securities that have been in a continuous

unrealized-loss

position for less than 12 months and for 12 or more months (in thousands):


	Less than 12 Months				12 Months or Longer				Total
June 30, 2020	Fair Value		Unrealized Loss		Fair Value		Unrealized Loss		Fair Value		Unrealized Loss

U.S. Treasury securities	$	5,055	$	1	$	—	$	—	$	5,055	$	1
Obligations of states and political subdivisions		18,249		50		—		—		18,249		50
Residential mortgage-backed securities		12,590		45		6,454		73		19,044		118
Commercial mortgage-backed securities		4,936		1		—		—		4,936		1


Total	$	40,830	$	97	$	6,454	$	73	$	47,284	$	170


	Less than 12 Months				12 Months or Longer				Total
June 30, 2019	Fair Value		Unrealized Loss		Fair Value		Unrealized Loss		Fair Value		Unrealized Loss

Obligations of states and political subdivisions	$	—	$	—	$	9,777	$	61	$	9,777	$	61
Residential mortgage-backed securities		17,594		77		404,946		3,024		422,540		3,101
Commercial mortgage-backed securities		—		—		134,881		459		134,881		459


Total	$	17,594	$	77	$	549,604	$	3,544	$	567,198	$	3,621


	Less than 12 Months				12 Months or Longer				Total
December 31, 2019	Fair Value		Unrealized Loss		Fair Value		Unrealized Loss		Fair Value		Unrealized Loss

Obligations of state and political subdivisions	$	65,787	$	400	$	326	$	—	$	66,113	$	400
Residential mortgage-backed securities		100,004		306		103,983		842		203,987		1,148
Commercial mortgage-backed securities		74,560		178		35,178		108		109,738		286

Total	$	240,351	$	884	$	139,487	$	950	$	379,838	$	1,834

The number of investments in an unrealized loss position totaled 14 at June 30, 2020. We do not believe these unrealized losses are “other-than-temporary” as (i) we do not have the intent to sell our securities prior to recovery and/or maturity and (ii) it is more likely than not that we will not have to sell our securities prior to recovery and/or maturity. In making this determination, we also consider the length of time and extent to which fair value has been less than cost and the financial condition of the issuer. The unrealized losses noted are interest rate related due to the level of interest rates at June 30, 2020 compared to the time of purchase. We have reviewed the ratings of the issuers and have not identified any issues related to the ultimate repayment of principal as a result of credit concerns on these securities. Our mortgage related securities are backed by GNMA, FNMA and FHLMC or are collateralized by securities backed by these agencies. At June 30, 2020, 88.76% of our

available-for-sale

securities that are obligations of states and political subdivisions were issued within the State of Texas, of which 51.14% are guaranteed by the Texas Permanent School Fund.

At June 30, 2020, $2,685,828,000 of

the Company’s securities were pledged as collateral for public or trust fund deposits, repurchase agreements, a borrowing line with the Federal Reserve Bank of Dallas and for other purposes required or permitted by law.

During the three months ended June 30, 2020 and 2019, sales of investment securities that were classified as

available-for-sale

totaled $157,521,000 and $65,821,000, respectively. Gross realized gains from security sales during the second quarter of 2020 and 2019 totaled $1,516,000 and $689,000, respectively. Gross realized losses from security sales during

the three-month period ended June 30,

020 and 2019 totaled $4,000 and $13,000, respectively.

During the six months ended June 30, 2020 and 2019, sales of investment securities classified as

available-for-sale

totaled $252,958,000 and $66,052,000, respectively. Gross realized gains from security sales during the

six-month

periods ended June 30, 2020 and 2019 totaled $3,578,000 and $693,000, respectively. Gross realized losses from security sales during the

six-month

periods ended June 30, 2020 and 2019 totaled $4,000 and $17,000, respectively.

The specific identification method was used to determine cost in order to compute the realized gains and losses.