2. INTEREST-BEARING TIME DEPOSITS IN BANKS AND SECURITIES:

Interest-bearing time deposits in banks totaled $1,458,000 at both December 31, 2018 and 2017 and at December 31, 2018, have original maturities within twelve months.

A summary of the Company’s available-for-sale securities as of December 31, 2018 and 2017 are as follows (dollars in thousands):


	December 31, 2018
	Amortized Cost Basis		Gross Unrealized Holding Gains		Gross Unrealized Holding Losses			Estimated Fair Value


Securities available-for-sale:
U.S. Treasury securities	$	9,970	$	—	$	(8	)	$	9,962
Obligations of U.S. government sponsored enterprises and agencies		301		—		—			301
Obligations of state and political subdivisions		1,229,828		30,013		(1,970	)		1,257,871
Corporate bonds and other		4,875		—		(77	)		4,798
Residential mortgage-backed securities		1,472,228		3,928		(21,611	)		1,454,545
Commercial mortgage-backed securities		436,366		670		(5,736	)		431,300

Total securities available-for-sale	$	3,153,568	$	34,611	$	(29,402	)	$	3,158,777


	December 31, 2017
	Amortized Cost Basis		Gross Unrealized Holding Gains		Gross Unrealized Holding Losses			Estimated Fair Value
Securities available-for-sale:
Obligations of U.S. government sponsored enterprises and agencies	$	60,516	$	—	$	(186	)	$	60,330
Obligations of state and political subdivisions		1,369,295		52,491		(936	)		1,420,850
Corporate bonds and other		11,421		43		(5	)		11,459
Residential mortgage-backed securities		1,223,452		4,561		(8,916	)		1,219,097
Commercial mortgage-backed securities		377,934		263		(2,460	)		375,737

Total securities available-for-sale	$	3,042,618	$	57,358	$	(12,503	)	$	3,087,473

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 December 31, 2018, were computed by using scheduled amortization of balances and historical prepayment rates. At December 31, 2018 and 2017, the Company did not hold any CMOs that entail higher risks than standard mortgage-backed securities.

The amortized cost and estimated fair value of available-for-sale securities at December 31, 2018, by contractual and expected maturity, are shown below (in thousands):


	Amortized Cost Basis		Estimated Fair Value
	Amortized Cost Basis		Estimated Fair Value
Due within one year	$	187,600	$	188,914
Due after one year through five years		547,868		563,679
Due after five years through ten years		507,792		518,317
Due after ten years		1,714		2,022
Mortgage-backed securities		1,908,594		1,885,845

Total	$	3,153,568	$	3,158,777

The following tables disclose, as of December 31, 2018 and 2017, 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
December 31, 2018	Fair Value		Unrealized Loss		Fair Value		Unrealized Loss		Fair Value		Unrealized Loss
U.S. Treasury securities	$	9,962	$	8	$	—	$	—	$	9,962	$	8
Obligations of U.S. government sponsored enterprises and agencies		—		—		301		—		301		—
Obligations of state and political subdivisions		27,489		107		114,461		1,863		141,950		1,970
Corporate bonds and other		4,348		68		450		9		4,798		77
Residential mortgage-backed securities		119,584		483		922,289		21,128		1,041,873		21,611
Commercial mortgage-backed securities		1,994		5		343,015		5,731		345,009		5,736

Total	$	163,377	$	671	$	1,380,516	$	28,731	$	1,543,893	$	29,402


	Less than 12 Months				12 Months or Longer				Total
December 31, 2017	Fair Value		Unrealized Loss		Fair Value		Unrealized Loss		Fair Value		Unrealized Loss
Obligations of U.S. government sponsored enterprises and agencies	$	60,329	$	186	$	—	$	—	$	60,329	$	186
Obligations of state and political subdivisions		66,361		219		44,938		717		111,299		936
Corporate bonds and other		224		2		237		3		461		5
Residential mortgage-backed securities		701,252		3,988		239,641		4,928		940,893		8,916
Commercial mortgage-backed securities		239,548		1,500		92,549		960		332,097		2,460

Total	$	1,067,714	$	5,895	$	377,365	$	6,608	$	1,445,079	$	12,503

The number of investments in an unrealized loss position totaled 366 at December 31, 2018. We do not believe these unrealized losses are “other-than-temporary”. In estimating other-than-temporary impairment losses, management considers, among other things, the length of time and the extent to which the fair value has been less than cost and the financial condition and near-term prospects of the issuer. Additionally management does not (i) 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 and (iii) that the length of time and extent that fair value has been less than cost is not indicative of recoverability. The unrealized losses noted are interest rate related due to the level of interest rates at December 31, 2018 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 December 31, 2018, 84.70% of our available-for-sale securities that are obligations of states and political subdivisions were issued within the State of Texas, of which 32.65% are guaranteed by the Texas Permanent School Fund.

Securities, carried at approximately $1,988,579,000 and $2,018,420,000 at December 31, 2018 and 2017, respectively, were pledged as collateral for public or trust fund deposits, repurchase agreements and for other purposes required or permitted by law.

During 2018, 2017 and 2016, sales of investment securities that were classified as available-for-sale totaled $220,259,000, $120,576,000 and $40,510,000. Gross realized gains from 2018, 2017 and 2016, securities sales were $1,847,000, $2,643,000 and $1,579,000, respectively. Gross realized losses from 2018, 2017 and 2016 securities sales were $493,000, $815,000 and $309,000, respectively. The specific identification method was used to determine cost in order to compute the realized gains and losses.