Fair Value Measurements (Details) (USD $)				3 Months Ended	12 Months Ended	6 Months Ended	12 Months Ended
	Jun. 30, 2013	Dec. 31, 2012	Jun. 30, 2012	Mar. 31, 2013 Sarepta Therapeutics, Inc.	Dec. 31, 2012 Sarepta Therapeutics, Inc.	Jun. 30, 2013 Regulus Therapeutics Inc.	Dec. 31, 2012 Regulus Therapeutics Inc.	Jun. 30, 2013 2 3/4 percent convertible senior notes	Jun. 30, 2013 Significant Other Observable Inputs (Level 2) 2 3/4 percent convertible senior notes	Jun. 30, 2013 Recurring basis Total	Dec. 31, 2012 Recurring basis Total	Jun. 30, 2013 Recurring basis Total Corporate debt securities	Dec. 31, 2012 Recurring basis Total Corporate debt securities	Jun. 30, 2013 Recurring basis Total Debt securities issued by U.S. government agencies	Dec. 31, 2012 Recurring basis Total Debt securities issued by U.S. government agencies	Jun. 30, 2013 Recurring basis Total Debt securities issued by the U.S. Treasury	Dec. 31, 2012 Recurring basis Total Debt securities issued by the U.S. Treasury	Jun. 30, 2013 Recurring basis Total Debt securities issued by states of the United States and political subdivisions of the states	Dec. 31, 2012 Recurring basis Total Debt securities issued by states of the United States and political subdivisions of the states	Jun. 30, 2013 Recurring basis Total Equity securities	Dec. 31, 2012 Recurring basis Total Equity securities	Jun. 30, 2013 Recurring basis Quoted Prices in Active Markets (Level 1)	Dec. 31, 2012 Recurring basis Quoted Prices in Active Markets (Level 1)	Jun. 30, 2013 Recurring basis Quoted Prices in Active Markets (Level 1) Debt securities issued by the U.S. Treasury	Dec. 31, 2012 Recurring basis Quoted Prices in Active Markets (Level 1) Debt securities issued by the U.S. Treasury	Jun. 30, 2013 Recurring basis Quoted Prices in Active Markets (Level 1) Equity securities	Dec. 31, 2012 Recurring basis Quoted Prices in Active Markets (Level 1) Equity securities	Jun. 30, 2013 Recurring basis Significant Other Observable Inputs (Level 2)	Dec. 31, 2012 Recurring basis Significant Other Observable Inputs (Level 2)	Jun. 30, 2013 Recurring basis Significant Other Observable Inputs (Level 2) Corporate debt securities	Dec. 31, 2012 Recurring basis Significant Other Observable Inputs (Level 2) Corporate debt securities	Jun. 30, 2013 Recurring basis Significant Other Observable Inputs (Level 2) Debt securities issued by U.S. government agencies	Dec. 31, 2012 Recurring basis Significant Other Observable Inputs (Level 2) Debt securities issued by U.S. government agencies	Jun. 30, 2013 Recurring basis Significant Other Observable Inputs (Level 2) Debt securities issued by states of the United States and political subdivisions of the states	Dec. 31, 2012 Recurring basis Significant Other Observable Inputs (Level 2) Debt securities issued by states of the United States and political subdivisions of the states	Jun. 30, 2013 Recurring basis Significant Unobservable Inputs (Level 3)	Dec. 31, 2012 Recurring basis Significant Unobservable Inputs (Level 3)	Dec. 31, 2012 Recurring basis Significant Unobservable Inputs (Level 3) Equity securities
Fair Value Measurements
Amount of transfer between Level 1 and Level 2 investments	$ 0		$ 0
Assets measured at fair value on a recurring basis
Cash equivalents										287,759,000	105,496,000											277,272,000	101,496,000					10,487,000	4,000,000
Available-for-sale securities												231,216,000	193,507,000	28,837,000	18,108,000	13,381,000	13,452,000	20,778,000	24,897,000	1,434,000	4,874,000			13,381,000	13,452,000	1,434,000	4,146,000			231,216,000	193,507,000	28,837,000	18,108,000	20,778,000	24,897,000			728,000
Net carrying value of investment	62,190,000	33,622,000			728,000	62,200,000	33,600,000			62,190,000	33,622,000																									62,190,000	33,622,000
Total assets										645,595,000	393,956,000											292,087,000	119,094,000					291,318,000	240,512,000							62,190,000	34,350,000
Realized gain on sale of common stock				1,100,000
Gross fair value of investment					1,000,000	69,200,000	44,400,000
Lack of marketability discount					296,000	7,000,000	10,800,000
Fair value of convertible notes									$ 357,200,000
Interest rate on convertible debt (as a percent)								2.75%