Fair Value Measurements, Assets Measured at Fair Value on a Recurring Basis (Details) (USD $)	Sep. 30, 2014		Dec. 31, 2013
Fair Value Measurements [Abstract]
Amount of transfer between Level 1 and Level 2 investments	$ 0
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment in Regulus Therapeutics Inc.	47,426,000		52,096,000
Interest rate on convertible debt (in hundredths)	2.75%		2.75%
2 3/4 percent convertible senior notes [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Interest rate on convertible debt (in hundredths)	2.75%
Fair value of convertible notes	490,000,000
Recurring basis [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	44,551,000		133,233,000
Investment in Regulus Therapeutics Inc.	47,426,000		52,096,000
Total assets	626,627,000		696,517,000
Recurring basis [Member] | Corporate debt securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	353,848,000	[1]	407,897,000	[1]
Recurring basis [Member] | Debt securities issued by U.S. government agencies [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	106,397,000	[1]	64,432,000
Recurring basis [Member] | Debt securities issued by the U.S. Treasury [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	9,040,000		15,328,000
Recurring basis [Member] | Debt securities issued by states of the United States and political subdivisions of the states [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	64,344,000	[1]	22,255,000
Recurring basis [Member] | Equity securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	1,021,000	[2]	1,276,000	[2]
Recurring basis [Member] | Quoted Prices in Active Markets (Level 1) [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	44,551,000		133,233,000
Investment in Regulus Therapeutics Inc.	47,426,000		52,096,000
Total assets	102,038,000		201,933,000
Recurring basis [Member] | Quoted Prices in Active Markets (Level 1) [Member] | Corporate debt securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	0	[1]	0	[1]
Recurring basis [Member] | Quoted Prices in Active Markets (Level 1) [Member] | Debt securities issued by U.S. government agencies [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	0	[1]	0
Recurring basis [Member] | Quoted Prices in Active Markets (Level 1) [Member] | Debt securities issued by the U.S. Treasury [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	9,040,000		15,328,000
Recurring basis [Member] | Quoted Prices in Active Markets (Level 1) [Member] | Debt securities issued by states of the United States and political subdivisions of the states [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	0	[1]	0
Recurring basis [Member] | Quoted Prices in Active Markets (Level 1) [Member] | Equity securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	1,021,000	[2]	1,276,000	[2]
Recurring basis [Member] | Significant Other Observable Inputs (Level 2) [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	0		0
Investment in Regulus Therapeutics Inc.	0		0
Total assets	524,589,000		494,584,000
Recurring basis [Member] | Significant Other Observable Inputs (Level 2) [Member] | Corporate debt securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	353,848,000	[1]	407,897,000	[1]
Recurring basis [Member] | Significant Other Observable Inputs (Level 2) [Member] | Debt securities issued by U.S. government agencies [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	106,397,000	[1]	64,432,000
Recurring basis [Member] | Significant Other Observable Inputs (Level 2) [Member] | Debt securities issued by the U.S. Treasury [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	0		0
Recurring basis [Member] | Significant Other Observable Inputs (Level 2) [Member] | Debt securities issued by states of the United States and political subdivisions of the states [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	64,344,000	[1]	22,255,000
Recurring basis [Member] | Significant Other Observable Inputs (Level 2) [Member] | Equity securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	0	[2]	0	[2]
Recurring basis [Member] | Significant Unobservable Inputs (Level 3) [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	0		0
Investment in Regulus Therapeutics Inc.	0		0
Total assets	0		0
Recurring basis [Member] | Significant Unobservable Inputs (Level 3) [Member] | Corporate debt securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	0	[1]	0	[1]
Recurring basis [Member] | Significant Unobservable Inputs (Level 3) [Member] | Debt securities issued by U.S. government agencies [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	0	[1]	0
Recurring basis [Member] | Significant Unobservable Inputs (Level 3) [Member] | Debt securities issued by the U.S. Treasury [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	0		0
Recurring basis [Member] | Significant Unobservable Inputs (Level 3) [Member] | Debt securities issued by states of the United States and political subdivisions of the states [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	0	[1]	0
Recurring basis [Member] | Significant Unobservable Inputs (Level 3) [Member] | Equity securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	$ 0	[2]	$ 0	[2]

[1] Includes investments classified as cash equivalents on our condensed consolidated balance sheet.

[2] Included in other current assets on our condensed consolidated balance sheet.