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


	September 30, 2011
	Amortized Cost Basis		Gross Unrealized Holding Gains		Gross Unrealized Holding Losses			Estimated Fair Value
Securities available-for-sale:
U. S. Treasury securities	$	15,171	$	253	$	—		$	15,424
Obligations of U.S. government sponsored-enterprises and agencies		244,429		7,092		—			251,521
Obligations of states and political subdivisions		590,453		42,555		(219	)		632,789
Corporate bonds and other		116,006		4,051		(168	)		119,889
Mortgage-backed securities		679,894		29,108		(68	)		708,934

Total securities available-for-sale	$	1,645,953	$	83,059	$	(455	)	$	1,728,557


Securities held-to-maturity:
Obligations of states and political subdivisions	$	3,920	$	45	$	—		$	3,965
Mortgage-backed securities		442		17		—			459

Total securities held-to-maturity	$	4,362	$	62	$	—		$	4,424


	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 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 September 30, 2011, were computed by using scheduled amortization of balances and historical prepayment rates. At September 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 September 30, 2011, by contractual and expected maturity, are shown below (in thousands):

During the quarter ended September 30, 2011 and 2010, sales of investment securities that were classified as available-for-sale totaled $800 thousand and $2.4 million, respectively. Gross realized gains from 2011 and 2010 securities sales and calls during the third quarter totaled $67 thousand and $7 thousand, respectively. There were no losses realized on securities sales and calls during these periods. During the nine-months ended September 30, 2011 and 2010, sales of investment securities that were classified as available-for-sale totaled $13.2 million and $17.4 million, respectively. Gross realized gains for 2011 and 2010 securities sales and calls during the nine-month period totaled $341 thousand and $79 thousand, respectively. Gross realized losses for the 2011 nine-month period totaled $13 thousand. There were no losses realized on securities sales during the 2010 nine-month period. The specific identification method was used to determine cost in order to compute the realized gains and losses.

The following tables disclose, as of September 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 for 12 or more months (in thousands):

	$166,507		$166,507		$166,507		$166,507	$166,507		$166,507
	Less than 12 Months			12 Months or Longer				Total
September 30, 2011	Fair Value	Unrealized Loss		Fair Value		Unrealized Loss		Fair Value	Unrealized Loss

Obligations of states and political subdivisions	$5,081	$	219		—		—	$5,081	$	219
Corporate bonds	18,506		168		—		—	18,506		168
Mortgage-backed securities	6,467		68		—		—	6,467		68

Total	$30,054	$	455	$	—	$	—	$30,054	$	455

		$166,507		$166,507		$166,507		$166,507		$166,507		$166,507

	Less than 12 Months				12 Months or Longer				Total
December 31, 2010	Fair Value		Unrealized Loss		Fair Value		Unrealized Loss		Fair Value		Unrealized Loss

Obligations of states and political subdivisions	$	164,437	$	5,665	$	2,070	$	196	$	166,507	$	5,861
Mortgage-backed securities		110,591		1,880		—		—		110,591		1,880

Total	$	275,028	$	7,545	$	2,070	$	196	$	277,098	$	7,741

The number of investment positions in an unrealized-loss position totaled 24 at September 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 September 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 $758.4 million at September 30, 2011, were pledged as collateral for public or trust fund deposits, repurchase agreements and for other purposes required or permitted by law.