Fair Value Measurements (Details) (USD $)	3 Months Ended	12 Months Ended
	Mar. 31, 2013	Dec. 31, 2012
Fair Value Measurements
Amount of transfer between Level 1 and Level 2 investments	$ 0
Assets measured at fair value on a recurring basis
Investment in Regulus Therapeutics Inc.	44,863,000	33,622,000
Sarepta Therapeutics, Inc.
Assets measured at fair value on a recurring basis
Realized gain on sale of common stock	1,100,000
Gross fair value of investment		1,000,000
Lack of marketability discount		296,000
Regulus Therapeutics Inc.
Assets measured at fair value on a recurring basis
Gross fair value of investment	54,600,000	44,400,000
Lack of marketability discount	9,700,000	10,800,000
2 3/4 percent convertible senior notes
Assets measured at fair value on a recurring basis
Interest rate on convertible debt (as a percent)	2.75%
Significant Other Observable Inputs (Level 2) | 2 3/4 percent convertible senior notes
Assets measured at fair value on a recurring basis
Fair value of convertible notes	258,000,000
Recurring basis | Total
Assets measured at fair value on a recurring basis
Cash equivalents	96,072,000	105,496,000
Investment in Regulus Therapeutics Inc.	44,863,000	33,622,000
Total assets	407,815,000	393,956,000
Recurring basis | Total | Corporate debt securities
Assets measured at fair value on a recurring basis
Available-for-sale securities	205,098,000	193,507,000
Recurring basis | Total | Debt securities issued by U.S. government agencies
Assets measured at fair value on a recurring basis
Available-for-sale securities	24,939,000	18,108,000
Recurring basis | Total | Debt securities issued by the U.S. Treasury
Assets measured at fair value on a recurring basis
Available-for-sale securities	13,424,000	13,452,000
Recurring basis | Total | Debt securities issued by states of the United States and political subdivisions of the states
Assets measured at fair value on a recurring basis
Available-for-sale securities	20,800,000	24,897,000
Recurring basis | Total | Equity securities
Assets measured at fair value on a recurring basis
Available-for-sale securities	2,619,000	4,874,000
Recurring basis | Quoted Prices in Active Markets (Level 1)
Assets measured at fair value on a recurring basis
Cash equivalents	91,072,000	101,496,000
Total assets	107,115,000	119,094,000
Recurring basis | Quoted Prices in Active Markets (Level 1) | Debt securities issued by the U.S. Treasury
Assets measured at fair value on a recurring basis
Available-for-sale securities	13,424,000	13,452,000
Recurring basis | Quoted Prices in Active Markets (Level 1) | Equity securities
Assets measured at fair value on a recurring basis
Available-for-sale securities	2,619,000	4,146,000
Recurring basis | Significant Other Observable Inputs (Level 2)
Assets measured at fair value on a recurring basis
Cash equivalents	5,000,000	4,000,000
Total assets	255,837,000	240,512,000
Recurring basis | Significant Other Observable Inputs (Level 2) | Corporate debt securities
Assets measured at fair value on a recurring basis
Available-for-sale securities	205,098,000	193,507,000
Recurring basis | Significant Other Observable Inputs (Level 2) | Debt securities issued by U.S. government agencies
Assets measured at fair value on a recurring basis
Available-for-sale securities	24,939,000	18,108,000
Recurring basis | Significant Other Observable Inputs (Level 2) | Debt securities issued by states of the United States and political subdivisions of the states
Assets measured at fair value on a recurring basis
Available-for-sale securities	20,800,000	24,897,000
Recurring basis | Significant Unobservable Inputs (Level 3)
Assets measured at fair value on a recurring basis
Investment in Regulus Therapeutics Inc.	44,863,000	33,622,000
Total assets	44,863,000	34,350,000
Recurring basis | Significant Unobservable Inputs (Level 3) | Equity securities
Assets measured at fair value on a recurring basis
Available-for-sale securities		$ 728,000