Investment Securities

The amortized cost and approximate fair values of investment securities are summarized as follows:



September 30, 2013	Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses			Estimated Fair Value
Available-for-Sale:
Obligations of U.S. government agencies	$	792,761	$	1,576	$	(9,862	)	$	784,475
U.S. agency residential mortgage-backed securities & collateralized mortgage obligations	1,157,281		14,836		(17,223		)	1,154,894
Private mortgage-backed securities	431		9		(3		)	437
Total	$	1,950,473	$	16,421	$	(27,088	)	$	1,939,806



September 30, 2013	Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses			Estimated Fair Value
Held-to-Maturity:
State, county and municipal securities	$	187,202	$	5,011	$	(1,861	)	$	190,352
Corporate securities	18,075		74		(19		)	18,130
Total	$	205,277	$	5,085	$	(1,880	)	$	208,482



December 31, 2012	Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses			Estimated Fair Value
Available-for-Sale:
Obligations of U.S. government agencies	$	751,501	$	3,518	$	(163	)	$	754,856
U.S. agency residential mortgage-backed securities & collateralized mortgage obligations	1,214,377		27,000		(1,526		)	1,239,851
Private mortgage-backed securities	539		13		(1		)	551
Total	$	1,966,417	$	30,531	$	(1,690	)	$	1,995,258



December 31, 2012	Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses			Estimated Fair Value
Held-to-Maturity:
State, county and municipal securities	$	192,875	$	10,835	$	(176	)	$	203,534
Corporate securities	14,975		64		(13		)	15,026
Other securities	373		—		—			373
Total	$	208,223	$	10,899	$	(189	)	$	218,933

Gross realized gains and losses from the disposition of investment securities are summarized in the following table:



	Three Months Ended September 30,				Nine Months Ended September 30,
	2013		2012		2013			2012
Gross realized gains	$	30	$	66	$	42		$	298
Gross realized losses	—		—		(16		)	(3		)

The following tables show the gross unrealized losses and fair values of investment securities, aggregated by investment category, and the length of time individual investment securities have been in a continuous unrealized loss position, as of September 30, 2013 and December 31, 2012



	Less than 12 Months					12 Months or More					Total
September 30, 2013	Fair Value		Gross Unrealized Losses			Fair Value		Gross Unrealized Losses			Fair Value		Gross Unrealized Losses
Available-for-Sale:
Obligations of U.S. government agencies	$	472,232	$	(9,862	)	$	—	$	—		$	472,232	$	(9,862	)
U.S. agency residential mortgage-backed securities & collateralized mortgage obligations	712,122		(17,223		)	—		—			712,122		(17,223		)
Private mortgage-backed securities	—		—			110		(3		)	110		(3		)
Total	$	1,184,354	$	(27,085	)	$	110	$	(3	)	$	1,184,464	$	(27,088	)



	Less than 12 Months					12 Months or More					Total
September 30, 2013	Fair Value		Gross Unrealized Losses			Fair Value		Gross Unrealized Losses			Fair Value		Gross Unrealized Losses
Held-to-Maturity:
State, county and municipal securities	$	51,606	$	(1,833	)	$	468	$	(28	)	$	52,074	$	(1,861	)
Corporate securities	7,312		(19		)	—		—			7,312		(19		)
Total	$	58,918	$	(1,852	)	$	468	$	(28	)	$	59,386	$	(1,880	)



	Less than 12 Months					12 Months or More					Total
December 31, 2012	Fair Value		Gross Unrealized Losses			Fair Value		Gross Unrealized Losses			Fair Value		Gross Unrealized Losses
Available-for-Sale:
Obligations of U.S. government agencies	$	93,982	$	(163	)	$	—	$	—		$	93,982	$	(163	)
U.S. agency residential mortgage-backed securities & collateralized mortgage obligations	250,198		(1,526		)	—		—			250,198		(1,526		)
Private mortgage-backed securities	—		—			137		(1		)	137		(1		)
Total	$	344,180	$	(1,689	)	$	137	$	(1	)	$	344,317	$	(1,690	)



	Less than 12 Months					12 Months or More					Total
December 31, 2012	Fair Value		Gross Unrealized Losses			Fair Value		Gross Unrealized Losses			Fair Value		Gross Unrealized Losses
Held-to-Maturity:
State, county and municipal securities	$	19,389	$	(168	)	$	557	$	(8	)	$	19,946	$	(176	)
Corporate securities	9,312		(13		)	—		—			9,312		(13		)
Total	$	28,701	$	(181	)	$	557	$	(8	)	$	29,258	$	(189	)

The investment portfolio is evaluated quarterly for other-than-temporary declines in the market value of each individual investment security. The Company had 211 and 69 individual investment securities that were in an unrealized loss position as of September 30, 2013 and December 31, 2012, respectively. Unrealized losses as of September 30, 2013 and December 31, 2012 related primarily to fluctuations in the current interest rates. The Company does not have the intent to sell any of the available-for-sale securities in the above table and it is not likely that the Company will have to sell any such securities before a recovery in cost. No impairment losses were recorded during the three or nine months ended September 30, 2013 and 2012.

Maturities of investment securities at September 30, 2013 are shown below. Maturities of mortgage-backed securities have been adjusted to reflect shorter maturities based upon estimated prepayments of principal. All other investment securities maturities are shown at contractual maturity dates.



	Available-for-Sale				Held-to-Maturity
September 30, 2013	Amortized Cost		Estimated Fair Value		Amortized Cost		Estimated Fair Value
Within one year	$	349,037	$	348,410	$	4,236	$	4,269
After one year but within five years	1,177,551		1,174,066		57,128		57,800
After five years but within ten years	377,332		370,872		85,444		87,378
After ten years	46,553		46,458		58,469		59,035
Total	$	1,950,473	$	1,939,806	$	205,277	$	208,482

As of September 30, 2013, the Company had investment securities callable within one year with amortized costs and estimated fair values of $380,305 and $374,729, respectively, including callable structured notes with amortized costs and estimated fair values of $123,286 and $122,087, respectively. These investment securities are primarily classified as available-for-sale and included in the after one year but within five years category in the table above.