9. Debt

Line of Credit

In September 2013, we entered into a credit agreement with Wells Fargo Bank, National Association (Wells Fargo) providing us a $75.0 million revolving loan facility, which may be increased by up to an additional $75.0 million provided certain conditions are met. At our option, amounts borrowed under this credit agreement bear interest at either the one-month LIBOR rate plus a 0.50 percent margin, or a fluctuating base rate excluding any margin. In addition, we are subject to a monthly commitment fee of 0.06 percent per annum on the average daily unused balance of the facility. In July 2015, we extended the term of this credit agreement to September 30, 2017. Amounts borrowed under the credit agreement are secured by certain of our marketable investments. As of June 30, 2015, we had no outstanding balance on the facility. This credit agreement does not contain any financial covenants.

Convertible Notes Due 2016

In October 2011, we issued $250.0 million in aggregate principal value 1.0 percent Convertible Senior Notes due September 15, 2016 (Convertible Notes). The Convertible Notes are unsecured, unsubordinated debt obligations that rank equally with all of our other unsecured and unsubordinated indebtedness. We pay interest semi-annually on March 15 and September 15 of each year. The initial conversion price is $47.69 per share and the number of underlying shares used to determine the aggregate consideration upon conversion is approximately 5.2 million shares.

At June 30, 2015, the aggregate conversion value of the Convertible Notes exceeded their par value by $91.3 million using a conversion price of $173.95, the closing price of our common stock on June 30, 2015.

During the three-month period ended June 30, 2015, we settled conversion requests representing $90.3 million in principal value of our Convertible Notes. We paid $90.3 million in principal and issued 1.4 million shares of our common stock during the settlement process. We received 1.4 million shares of our common stock under our note hedge from Deutsche Bank AG London (DB London) which we placed into our treasury stock account. We recognized a $2.8 million extinguishment loss with the settlement of these conversions. As of June 30, 2015, there were 723,000 shares of our common stock that could be issued upon future conversions of our outstanding Convertible Notes.

We have received additional conversion requests representing $2.9 million in principal value of our Convertible Notes, which will settle in the third quarter of 2015. Based on our stock price as of June 30, 2015, we expect that the full $2.9 million of principal will be paid in cash to the converting note holders, and that we will be required to issue shares of our common stock. We expect to receive an equal number of shares of our common stock under our note hedge with DB London and to recognize an estimated $66,000 extinguishment loss upon settlement of these conversions.

Interest expense incurred in connection with our Convertible Notes consisted of the following (in thousands):

	Three Months Ended June 30,				Six Months Ended June 30,
	2015		2014		2015		2014
Contractual coupon rate of interest	$	83	$	625	$	327	$	1,250
Discount amortization	824		2,973		2,422		5,867

Interest expense—convertible notes	$	907	$	3,598	$	2,749	$	7,117

Components comprising the carrying value of the Convertible Notes include the following (in thousands):

	June 30, 2015			December 31, 2014
Principal balance	$	34,484		$	138,750
Discount, net of accumulated amortization of $5,789 and $19,819	(2,202		)	(12,336		)

Carrying amount	$	32,282		$	126,414