Investment Securities

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



March 31, 2015	Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses			Estimated Fair Value
Available-for-Sale:
Obligations of U.S. government agencies	$	769,260	$	2,015	$	(1,820	)	$	769,455
U.S. agency residential mortgage-backed securities & collateralized mortgage obligations	993,459		15,702		(1,861		)	1,007,300
Private mortgage-backed securities	300		4		(2		)	302
Total	$	1,763,019	$	17,721	$	(3,683	)	$	1,777,057



March 31, 2015	Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses			Estimated Fair Value
Held-to-Maturity:
State, county and municipal securities	$	188,386	$	6,613	$	(190	)	$	194,809
Corporate securities	39,531		150		(17		)	39,664
U.S agency residential mortgage-backed securities & collateralized mortgage obligations	335,474		8,762		(2,235		)	342,001
Other investments	456		1		—			457
Total	$	563,847	$	15,526	$	(2,442	)	$	576,931



December 31, 2014	Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses			Estimated Fair Value
Available-for-Sale:
Obligations of U.S. government agencies	$	725,408	$	895	$	(5,370	)	$	720,933
U.S. agency residential mortgage-backed securities & collateralized mortgage obligations	982,764		11,526		(3,624		)	990,666
Private mortgage-backed securities	322		5		(2		)	325
Total	$	1,708,494	$	12,426	$	(8,996	)	$	1,711,924



December 31, 2014	Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses			Estimated Fair Value
Held-to-Maturity:
State, county and municipal securities	$	188,941	$	5,949	$	(386	)	$	194,504
Corporate securities	32,565		54		(75		)	32,544
U.S. agency residential mortgage-backed securities & collateralized mortgage obligations	353,176		5,563		(1,758		)	356,981
Other Investments	504		—		—			504
Total	$	575,186	$	11,566	$	(2,219	)	$	584,533

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



	Three Months Ended March 31,
	2015		2014
Gross realized gains	$	6	$	225
Gross realized losses	—		154

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 March 31, 2015 and December 31, 2014.



	Less than 12 Months					12 Months or More					Total
March 31, 2015	Fair Value		Gross Unrealized Losses			Fair Value		Gross Unrealized Losses			Fair Value		Gross Unrealized Losses
Available-for-Sale:
Obligations of U.S. government agencies	$	76,190	$	(185	)	$	277,320	$	(1,635	)	$	353,510	$	(1,820	)
U.S. agency residential mortgage-backed securities & collateralized mortgage obligations	23,948		(56		)	142,213		(1,805		)	166,161		(1,861		)
Private mortgage-backed securities	—		—			84		(2		)	84		(2		)
Total	$	100,138	$	(241	)	$	419,617	$	(3,442	)	$	519,755	$	(3,683	)



	Less than 12 Months					12 Months or More					Total
March 31, 2015	Fair Value		Gross Unrealized Losses			Fair Value		Gross Unrealized Losses			Fair Value		Gross Unrealized Losses
Held-to-Maturity:
State, county and municipal securities	$	1,778	$	(10	)	$	13,831	$	(180	)	$	15,609	$	(190	)
U.S. agency residential mortgage-backed securities & collateralized mortgage obligations	45,264		(2,235		)	—		—			45,264		(2,235		)
Corporate Securities	10,046		(17		)	—		—			10,046		(17		)
Total	$	57,088	$	(2,262	)	$	13,831	$	(180	)	$	70,919	$	(2,442	)



	Less than 12 Months					12 Months or More					Total
December 31, 2014	Fair Value		Gross Unrealized Losses			Fair Value		Gross Unrealized Losses			Fair Value		Gross Unrealized Losses
Available-for-Sale:
Obligations of U.S. government agencies	$	135,888	$	(702	)	$	309,283	$	(4,668	)	$	445,171	$	(5,370	)
U.S. agency residential mortgage-backed securities & collateralized mortgage obligations	219,214		(887		)	151,380		(2,737		)	370,594		(3,624		)
Private mortgage-backed securities	—		—			90		(2		)	90		(2		)
Total	$	355,102	$	(1,589	)	$	460,753	$	(7,407	)	$	815,855	$	(8,996	)



	Less than 12 Months					12 Months or More					Total
December 31, 2014	Fair Value		Gross Unrealized Losses			Fair Value		Gross Unrealized Losses			Fair Value		Gross Unrealized Losses
Held-to-Maturity:
State, county and municipal securities	$	7,979	$	(13	)	$	20,097	$	(373	)	$	28,076	$	(386	)
U.S. agency residential mortgage-backed securities & collateralized mortgage obligations	61,201		(1,758		)	—		—			61,201		(1,758		)
Corporate securities	14,755		(75		)	—		—			14,755		(75		)
Total	$	83,935	$	(1,846	)	$	20,097	$	(373	)	$	104,032	$	(2,219	)

The investment portfolio is evaluated quarterly for other-than-temporary declines in the market value of each individual investment security. The Company had 100 and 154 individual investment securities that were in an unrealized loss position as of March 31, 2015 and December 31, 2014, respectively. Unrealized losses as of March 31, 2015 and December 31, 2014 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 months ended March 31, 2015 or 2014.

Maturities of investment securities at March 31, 2015 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
March 31, 2015	Amortized Cost		Estimated Fair Value		Amortized Cost		Estimated Fair Value
Within one year	$	288,960	$	292,432	$	79,106	$	80,955
After one year but within five years	1,316,177		1,325,048		286,575		292,101
After five years but within ten years	126,241		127,514		133,958		137,489
After ten years	31,641		32,063		64,208		66,386
Total	$	1,763,019	$	1,777,057	$	563,847	$	576,931

As of March 31, 2015, the Company had investment securities callable within one year with amortized costs and estimated fair values of $166,196 and $166,636, respectively, including callable structured notes with amortized costs and estimated fair values of $29,996 and $30,030, 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.