Cash and Cash Equivalents and Investments in Marketable Securities

Our investment portfolio at September 30, 2015 consisted of cash, cash equivalents and investments in marketable securities.

We currently invest in marketable debt securities with the maximum maturity or put date of any individual issue generally not greater than one year from the date of purchase, which are usually held until maturity. All of these instruments are classified as available-for-sale. As a result, they are reported at fair value using quoted market prices. Interest income is recorded as earned. Unrealized gains and losses, net of related income taxes, are reported as a component of accumulated other comprehensive income. Upon sale or maturity, realized gains on our marketable debt securities are recognized as interest income. These gains are computed based on the specific identification of the underlying cost of the securities, net of unrealized gains and losses previously reported in other comprehensive income. Unrealized gains and losses on our available-for-sale securities are due to changes in market prices and are considered temporary.

The following is a summary of our marketable securities:



	Amortized Cost		Gross Unrealized Gain		Gross Unrealized Loss			Fair Value (Net Carrying Amount)
	(In thousands)
September 30, 2015
Certificates of deposit	$	5,999	$	4	$	—		$	6,003
Commercial paper	58,299		21		—			58,320
Corporate debt securities	86,657		13		(31		)	86,639
State and political subdivisions debt securities	236,075		58		(42		)	236,091
Total marketable securities	$	387,030	$	96	$	(73	)	$	387,053

December 31, 2014
Certificates of deposit	$	54,000	$	10	$	—		$	54,010
Commercial paper	52,297		7		(4		)	52,300
Corporate debt securities	136,181		1		(94		)	136,088
State and political subdivisions debt securities	231,819		5		(112		)	231,712
Total marketable securities	$	474,297	$	23	$	(210	)	$	474,110

Interest income recognized on our marketable securities was $0.5 million for both the three months ended September 30, 2015 and 2014, and $1.4 million and $1.7 million for the nine months ended September 30, 2015 and 2014, respectively.