Fair Value Measurements (Details) (USD $)				3 Months Ended	12 Months Ended	9 Months Ended	12 Months Ended
	Sep. 30, 2013	Dec. 31, 2012	Sep. 30, 2012	Mar. 31, 2013 Sarepta Therapeutics, Inc.	Dec. 31, 2012 Sarepta Therapeutics, Inc.	Sep. 30, 2013 Regulus Therapeutics Inc.	Dec. 31, 2012 Regulus Therapeutics Inc.	Sep. 30, 2013 2 3/4 percent convertible senior notes	Sep. 30, 2013 Significant Other Observable Inputs (Level 2) 2 3/4 percent convertible senior notes	Sep. 30, 2013 Recurring basis Total	Dec. 31, 2012 Recurring basis Total	Sep. 30, 2013 Recurring basis Total Corporate debt securities	Dec. 31, 2012 Recurring basis Total Corporate debt securities	Sep. 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	Sep. 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	Sep. 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	Sep. 30, 2013 Recurring basis Total Equity securities	Dec. 31, 2012 Recurring basis Total Equity securities	Sep. 30, 2013 Recurring basis Quoted Prices in Active Markets (Level 1)	Dec. 31, 2012 Recurring basis Quoted Prices in Active Markets (Level 1)	Sep. 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	Sep. 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	Sep. 30, 2013 Recurring basis Significant Other Observable Inputs (Level 2)	Dec. 31, 2012 Recurring basis Significant Other Observable Inputs (Level 2)	Sep. 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	Sep. 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	Sep. 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	Sep. 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										241,007,000	105,496,000											239,407,000	101,496,000					1,600,000	4,000,000
Available-for-sale securities												337,967,000	193,507,000	36,820,000	18,108,000	16,364,000	13,452,000	23,339,000	24,897,000	1,832,000	4,874,000			16,364,000	13,452,000	1,832,000	4,146,000			337,967,000	193,507,000	36,820,000	18,108,000	23,339,000	24,897,000			728,000
Net carrying value of investment	65,004,000	33,622,000			728,000	65,000,000	33,600,000			65,004,000	33,622,000																									65,004,000	33,622,000
Total assets										722,333,000	393,956,000											257,603,000	119,094,000					399,726,000	240,512,000							65,004,000	34,350,000
Realized gain on sale of common stock				1,100,000
Gross fair value of investment					1,000,000	66,500,000	44,400,000
Lack of marketability discount					296,000	1,500,000	10,800,000
Fair value of convertible notes									$ 480,600,000
Interest rate on convertible debt (as a percent)								2.75%