Note 3 - Investment Securities

The amortized cost and estimated fair values of investments in debt and equity securities are summarized in the following tables:


	June 30, 2016
	Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses			Estimated Fair Value
			(in thousands)
Securities Available for Sale
Obligations of U.S. government corporations and agencies	$	398,811	$	10,175		—		$	408,986
Obligations of states and political subdivisions		111,763		5,221		—			116,984
Marketable equity securities		3,000		47		—			3,047


Total securities available for sale	$	513,574	$	15,443		—		$	529,017


Securities Held to Maturity
Obligations of U.S. government corporations and agencies	$	659,867	$	24,532		—		$	684,399
Obligations of states and political subdivisions		14,545		655		—			15,200


Total securities held to maturity	$	674,412	$	25,187		—		$	699,599


	December 31, 2015
	Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses			Estimated Fair Value
			(in thousands)
Securities Available for Sale
Obligations of U.S. government corporations and agencies	$	312,917	$	2,761	$	(1,996	)	$	313,682
Obligations of states and political subdivisions		86,823		1,428		(33	)		88,218
Marketable equity securities		3,000		—		(15	)		2,985


Total securities available for sale	$	402,740	$	4,189	$	(2,044	)	$	404,885


Securities Held to Maturity
Obligations of U.S. government corporations and agencies	$	711,994	$	8,394	$	(2,882	)	$	717,506
Obligations of states and political subdivisions		14,536		277		(110	)		14,703


Total securities held to maturity	$	726,530	$	8,671	$	(2,992	)	$	732,209

No investment securities were sold during the six months ended June 30, 2016 or the six months ended June 30, 2015. Investment securities with an aggregate carrying value of $277,023,000 and $297,547,000 at June 30, 2016 and December 31, 2015, respectively, were pledged as collateral for specific borrowings, lines of credit and local agency deposits.

The amortized cost and estimated fair value of debt securities at June 30, 2016 by contractual maturity are shown below. Actual maturities may differ from contractual maturities because borrowers may have the right to call or prepay obligations with or without call or prepayment penalties. At June 30, 2016, obligations of U.S. government corporations and agencies with a cost basis totaling $1,058,678,000 consist almost entirely of mortgage-backed securities whose contractual maturity, or principal repayment, will follow the repayment of the underlying mortgages. For purposes of the following table, the entire outstanding balance of these mortgage-backed securities issued by U.S. government corporations and agencies is categorized based on final maturity date. At June 30, 2016, the Company estimates the average remaining life of these mortgage-backed securities issued by U.S. government corporations and agencies to be approximately 3.9 years. Average remaining life is defined as the time span after which the principal balance has been reduced by half.


Investment Securities	Available for Sale				Held to Maturity
(In thousands)	Amortized Cost		Estimated Fair Value		Amortized Cost		Estimated Fair Value
Due in one year	$	1	$	1		—		—
Due after one year through five years		12,246		12,748	$	1,161	$	1,201
Due after five years through ten years		16,676		17,557		845		936
Due after ten years		484,651		498,711		672,406		697,462

Totals	$	513,574	$	529,017	$	674,412	$	699,599

Gross unrealized losses on investment securities and the fair value of the related securities, aggregated by investment category and length of time that individual securities have been in a continuous unrealized loss position, were as follows:


	Less than 12 months					12 months or more					Total
June 30, 2016	Fair Value		Unrealized Loss			Fair Value		Unrealized Loss			Fair Value		Unrealized Loss
	(in thousands)
Securities Available for Sale:
Obligations of U.S. government corporations and agencies		—		—			—		—			—		—
Obligations of states and political subdivisions		—		—			—		—			—		—
Marketable equity securities		—		—			—		—			—		—


Total securities available-for-sale		—		—			—		—			—		—


Securities Held to Maturity:
Obligations of U.S. government corporations and agencies		—		—			—		—			—		—
Obligations of states and political subdivisions		—		—			—		—			—		—


Total securities held-to-maturity		—		—			—		—			—		—


	Less than 12 months					12 months or more					Total
December 31, 2015	Fair Value		Unrealized Loss			Fair Value		Unrealized Loss			Fair Value		Unrealized Loss
	(in thousands)
Securities Available for Sale:
Obligations of U.S. government corporations and agencies	$	193,306	$	(1,996	)		—		—		$	193,306	$	(1,996	)
Obligations of states and political subdivisions		6,469		(33	)		—		—			6,469		(33	)
Marketable equity securities		2,985		(15	)		—		—			2,985		(15	)


Total securities available-for-sale	$	202,760	$	(2,044	)		—		—		$	202,760	$	(2,044	)


Securities Held to Maturity:
Obligations of U.S. government corporations and agencies	$	198,481	$	(2,882	)		—		—		$	198,481	$	(2,882	)
Obligations of states and political subdivisions		497		(11	)	$	1,121	$	(99	)		1,618		(110	)


Total securities held-to-maturity	$	198,978	$	(2,893	)	$	1,121	$	(99	)	$	200,099	$	(2,992	)

Obligations of U.S. government corporations and agencies: Unrealized losses on investments in obligations of U.S. government corporations and agencies are caused by interest rate increases. The contractual cash flows of these securities are guaranteed by U.S. Government Sponsored Entities (principally Fannie Mae and Freddie Mac). It is expected that the securities would not be settled at a price less than the amortized cost of the investment. Because the decline in fair value is attributable to changes in interest rates and not credit quality, and because the Company does not intend to sell and more likely than not will not be required to sell, these investments are not considered other-than-temporarily impaired. At June 30, 2016, no debt securities representing obligations of U.S. government corporations and agencies had unrealized losses.

Obligations of states and political subdivisions: The unrealized losses on investments in obligations of states and political subdivisions were caused by increases in required yields by investors in these types of securities. It is expected that the securities would not be settled at a price less than the amortized cost of the investment. Because the decline in fair value is attributable to changes in interest rates and not credit quality, and because the Company does not intend to sell and more likely than not will not be required to sell, these investments are not considered other-than-temporarily impaired. At June 30, 2016, no debt securities representing obligations of states and political subdivisions had unrealized losses.

Marketable equity securities: At June 30, 2016, no marketable equity securities had unrealized losses.