Investment Securities

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



March 31, 2014	Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses			Estimated Fair Value
Available-for-Sale:
Obligations of U.S. government agencies	$	712,895	$	1,563	$	(7,866	)	$	706,592
U.S. agency residential mortgage-backed securities & collateralized mortgage obligations	1,190,134		13,316		(16,779		)	1,186,671
Private mortgage-backed securities	384		7		(2		)	389
Total	$	1,903,413	$	14,886	$	(24,647	)	$	1,893,652



March 31, 2014	Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses			Estimated Fair Value
Held-to-Maturity:
State, county and municipal securities	$	183,472	$	5,101	$	(1,209	)	$	187,364
Corporate securities	17,964		115		—			18,079
Total	$	201,436	$	5,216	$	(1,209	)	$	205,443



December 31, 2013	Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses			Estimated Fair Value
Available-for-Sale:
Obligations of U.S. government agencies	$	774,055	$	1,432	$	(12,249	)	$	763,238
U.S. agency residential mortgage-backed securities & collateralized mortgage obligations	1,197,295		11,905		(25,147		)	1,184,053
Private mortgage-backed securities	407		9		(1		)	415
Total	$	1,971,757	$	13,346	$	(37,397	)	$	1,947,706



December 31, 2013	Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses			Estimated Fair Value
Held-to-Maturity:
State, county and municipal securities	$	185,818	$	4,043	$	(2,049	)	$	187,812
Corporate securities	18,019		103		(8		)	18,114
Total	$	203,837	$	4,146	$	(2,057	)	$	205,926

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



	Three Months Ended March 31,
	2014		2013
Gross realized gains	$	225	$	8
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, 2014 and December 31, 2013



	Less than 12 Months					12 Months or More					Total
March 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	$	289,936	$	(4,862	)	$	143,933	$	(3,004	)	$	433,869	$	(7,866	)
U.S. agency residential mortgage-backed securities & collateralized mortgage obligations	427,631		(6,268		)	304,999		(10,511		)	732,630		(16,779		)
Private mortgage-backed securities	—		—			100		(2		)	100		(2		)
Total	$	717,567	$	(11,130	)	$	449,032	$	(13,517	)	$	1,166,599	$	(24,647	)



	Less than 12 Months					12 Months or More					Total
March 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	$	21,088	$	(473	)	$	21,777	$	(736	)	$	42,865	$	(1,209	)
Total	$	21,088	$	(473	)	$	21,777	$	(736	)	$	42,865	$	(1,209	)



	Less than 12 Months					12 Months or More					Total
December 31, 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	$	458,385	$	(10,355	)	$	59,362	$	(1,894	)	$	517,747	$	(12,249	)
U.S. agency residential mortgage-backed securities & collateralized mortgage obligations	634,199		(17,273		)	166,930		(7,874		)	801,129		(25,147		)
Private mortgage-backed securities	—		—			104		(1		)	104		(1		)
Total	$	1,092,584	$	(27,628	)	$	226,396	$	(9,769	)	$	1,318,980	$	(37,397	)



	Less than 12 Months					12 Months or More					Total
December 31, 2013	Fair Value		Gross Unrealized Losses			Fair Value		Gross Unrealized Losses			Fair Value		Gross Unrealized Losses
Held-to-Maturity:
State, county and municipal securities	$	37,550	$	(1,319	)	$	14,296	$	(730	)	$	51,846	$	(2,049	)
Corporate securities	7,294		(8		)	—		—			7,294		(8		)
Total	$	44,844	$	(1,327	)	$	14,296	$	(730	)	$	59,140	$	(2,057	)

The investment portfolio is evaluated quarterly for other-than-temporary declines in the market value of each individual investment security. The Company had 198 and 229 individual investment securities that were in an unrealized loss position as of March 31, 2014 and December 31, 2013, respectively. Unrealized losses as of March 31, 2014 and December 31, 2013 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, 2014 and 2013.

Maturities of investment securities at March 31, 2014 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, 2014	Amortized Cost		Estimated Fair Value		Amortized Cost		Estimated Fair Value
Within one year	$	249,676	$	249,281	$	6,992	$	7,067
After one year but within five years	1,200,266		1,195,368		59,141		59,905
After five years but within ten years	393,945		389,650		84,471		86,512
After ten years	59,526		59,353		50,832		51,959
Total	$	1,903,413	$	1,893,652	$	201,436	$	205,443

As of March 31, 2014, the Company had investment securities callable within one year with amortized costs and estimated fair values of $200,270 and $198,596, respectively, including callable structured notes with amortized costs and estimated fair values of $54,985 and $54,994, 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.