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Cash, Cash Equivalents and Available for Sale Securities (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014	Dec. 31, 2014
Cash, Cash Equivalents and Available for Sale Securities
Cash	$ 1,800		$ 1,800		$ 4,700
Investments in U.S. Treasury money market funds and U.S. Treasury securities with maturities less than 90 days	253,100		253,100		35,100
Available for sale securities, current	26,147		26,147		423,746
Available for sale securities, noncurrent	144,944		144,944
Available for sale securities, including carrying value and estimated fair values
Cost	170,952		170,952		423,859
Unrealized Gain (Loss)	138		138		(113)
Changes in accumulated other comprehensive loss
Beginning balance	24	$ 25	(65)
Current period changes in fair value before reclassifications, net of tax	85	10	174	$ 35
Amounts reclassified from accumulated other comprehensive income, net of tax	(30)		(30)
Other comprehensive income	55	10	144	35
Ending balance	79	$ 35	79	$ 35
Fair Value
Available for sale securities, including carrying value and estimated fair values
Value	171,090		171,090		423,746
Carrying Value
Available for sale securities, including carrying value and estimated fair values
Value	171,090		171,090		423,746
U.S. Treasury securities
Available for sale securities, including carrying value and estimated fair values
Cost	106,340		106,340		423,859
Unrealized Gain (Loss)	124		124		(113)
U.S. Treasury securities \| Fair Value
Available for sale securities, including carrying value and estimated fair values
Value	106,464		106,464		423,746
U.S. Treasury securities \| Carrying Value
Available for sale securities, including carrying value and estimated fair values
Value	106,464		106,464		$ 423,746
Corporate debt securities
Available for sale securities, including carrying value and estimated fair values
Cost	64,612		64,612
Unrealized Gain (Loss)	14		14
Corporate debt securities \| Fair Value
Available for sale securities, including carrying value and estimated fair values
Value	64,626		64,626
Corporate debt securities \| Carrying Value
Available for sale securities, including carrying value and estimated fair values
Value	$ 64,626		$ 64,626