A summary of available-for-sale and held-to-maturity securities follows (in thousands):


	June 30, 2011
	Amortized Cost Basis		Gross Unrealized Holding Gains		Gross Unrealized Holding Losses			Estimated Fair Value
Securities available-for-sale:
U. S. Treasury securities	$	15,198	$	271	$	—		$	15,469
Obligations of U.S. government sponsored-enterprises and agencies		251,648		7,585		—			259,233
Obligations of states and political subdivisions		555,118		28,944		(489	)		583,573
Corporate bonds and other		41,164		4,058		—			45,222
Mortgage-backed securities		710,781		26,259		(545	)		736,495

Total securities available-for-sale	$	1,573,909	$	67,117	$	(1,034	)	$	1,639,992


Securities held-to-maturity:
Obligations of states and political subdivisions	$	6,201	$	76	$	—		$	6,277
Mortgage-backed securities		462		17		—			479

Total debt securities held-to-maturity	$	6,663	$	93	$	—		$	6,756


	December 31, 2010
	Amortized Cost Basis		Gross Unrealized Holding Gains		Gross Unrealized Holding Losses			Estimated Fair Value
Securities available-for-sale:
U. S. Treasury securities	$	15,253	$	263	$	—		$	15,516
Obligations of U.S. government sponsored-enterprises and agencies		270,706		8,542		—			279,248
Obligations of states and political subdivisions		543,074		12,695		(5,861	)		549,908
Corporate bonds and other		56,710		4,118		—			60,828
Mortgage-backed securities		611,275		22,283		(1,880	)		631,678

Total securities available-for-sale	$	1,497,018	$	47,901	$	(7,741	)	$	1,537,178


Securities held-to-maturity:
Obligations of states and political subdivisions	$	8,549	$	160	$	—		$	8,709
Mortgage-backed securities		515		16		—			531

Total debt securities held-to-maturity	$	9,064	$	176	$	—		$	9,240

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, 2011, were computed by using scheduled amortization of balances and historical prepayment rates. At June 30, 2011 and December 31, 2010, the Company did not hold any CMOs that entail higher risks than standard mortgage-backed securities.

The amortized cost and estimated fair value of debt securities at June 30, 2011, by contractual and expected maturity, are shown below (in thousands):

During the quarter ended June 30, 2011 and 2010, sales of investment securities that were classified as available-for-sale totaled $1.2 million and $11.8 million, respectively. Gross realized gains from 2011 and 2010 securities sales and calls during the second quarter totaled $43 thousand and $72 thousand, respectively. Gross realized losses from sales during the second quarter of 2011 totaled $1 thousand. There were no losses realized on securities sales during the second quarter of 2010. During the six-months ended June 30, 2011 and 2010, sales of investment securities that were classified as available-for-sale totaled $12.4 million and $15.0 million, respectively. Gross realized gains for 2011 and 2010 securities sales and calls during the six-month period totaled $273 thousand and $72 thousand, respectively. Gross realized losses from sales during the six-month period ended June 30, 2011 totaled $13 thousand. There were no losses realized on securities sales during the same period in 2010. The specific identification method was used to determine cost in order to compute the realized gains.

The following tables disclose, as of June 30, 2011 and December 31, 2010, our available-for-sale and held-to-maturity securities that have been in a continuous unrealized-loss position for less than 12 months and those that have been in a continuous unrealized-loss position for 12 or more months (in thousands):

The number of investment positions in this unrealized-loss position totaled 54 at June 30, 2011. 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 the 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, 2011 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.

Securities, carried at approximately $793.2 million at June 30, 2011, were pledged as collateral for public or trust fund deposits, repurchase agreements and for other purposes required or permitted by law.