Cash and Cash Equivalents and Investments in Marketable Securities

Our investment portfolio at September 30, 2013 consisted of cash, cash equivalents and investments in marketable debt securities.

We invest in highly-rated marketable debt securities that have maturities at the date of purchase of greater than three months. We also invest in other marketable debt securities with the maximum maturity or put date of any individual issue generally not greater than two years 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 available-for-sale securities:



	Amortized Cost		Gross Unrealized Gain		Gross Unrealized Loss			Fair Value (Net Carrying Amount)
	(In thousands)
September 30, 2013
Certificates of deposit	$	69,756	$	16	$	(4	)	$	69,768
Commercial paper	40,237		10		—			40,247
Corporate debt securities	66,791		8		(16		)	66,783
State and political subdivisions debt securities	485,394		34		(62		)	485,366
Total marketable securities	$	662,178	$	68	$	(82	)	$	662,164

December 31, 2012
Certificates of deposit	$	82,791	$	14	$	(6	)	$	82,799
Commercial paper	45,737		17		—			45,754
Corporate debt securities	49,587		2		(30		)	49,559
State and political subdivisions debt securities	457,615		26		(51		)	457,590
Total marketable securities	$	635,730	$	59	$	(87	)	$	635,702

Interest income recognized on our marketable debt securities was $0.5 million and $0.2 million for the three months ended September 30, 2013 and 2012, respectively, and $1.6 million and $0.7 million for the nine months ended September 30, 2013 and 2012, respectively.