Fair Value Measurements, Assets Measured at Fair Value on a Recurring Basis (Details) - USD ($) $ in Thousands		Sep. 30, 2015		Dec. 31, 2014		Sep. 30, 2014
Fair Value Measurements [Abstract]
Transfers from Level 1 to Level 2		$ 0
Transfers from Level 2 to Level 1		0
Fair Value Measurements [Abstract]
Available-for-sale securities		711,793		$ 687,788
Investment in Regulus Therapeutics Inc.		$ 18,594		$ 81,881
2 3/4 Percent Convertible Senior Notes [Member]
Fair Value Measurements [Abstract]
Interest rate on convertible senior notes		2.75%		2.75%		2.75%
Fair value of convertible notes		$ 149,400
1 Percent Convertible Senior Notes [Member]
Fair Value Measurements [Abstract]
Interest rate on convertible senior notes		1.00%		1.00%
Fair value of convertible notes		$ 454,600
Recurring Basis [Member]
Fair Value Measurements [Abstract]
Cash equivalents	[1]	114,602		$ 104,680
Investment in Regulus Therapeutics Inc.		18,594		81,881
Total assets		826,395		792,468
Recurring Basis [Member] | Corporate Debt Securities [Member]
Fair Value Measurements [Abstract]
Available-for-sale securities		447,597	[2]	372,002	[3]
Recurring Basis [Member] | Corporate Debt Securities [Member] | Cash and Cash Equivalents [Member]
Fair Value Measurements [Abstract]
Available-for-sale securities				800
Recurring Basis [Member] | Debt Securities issued by U.S. Government Agencies [Member]
Fair Value Measurements [Abstract]
Available-for-sale securities	[2]	122,282		109,855
Recurring Basis [Member] | Debt Securities issued by the U.S. Treasury [Member]
Fair Value Measurements [Abstract]
Available-for-sale securities	[4]			19,017
Recurring Basis [Member] | Debt Securities issued by the U.S. Treasury [Member] | Cash and Cash Equivalents [Member]
Fair Value Measurements [Abstract]
Available-for-sale securities				10,000
Recurring Basis [Member] | Debt Securities issued by States of the United States and Political Subdivisions of the States [Member]
Fair Value Measurements [Abstract]
Available-for-sale securities		123,320	[5]	105,033	[6]
Recurring Basis [Member] | Debt Securities issued by States of the United States and Political Subdivisions of the States [Member] | Cash and Cash Equivalents [Member]
Fair Value Measurements [Abstract]
Available-for-sale securities		7,600		9,300
Recurring Basis [Member] | Quoted Prices in Active Markets (Level 1) [Member]
Fair Value Measurements [Abstract]
Cash equivalents	[1]	114,602		104,680
Investment in Regulus Therapeutics Inc.		18,594		0
Total assets		133,196		123,697
Recurring Basis [Member] | Quoted Prices in Active Markets (Level 1) [Member] | Corporate Debt Securities [Member]
Fair Value Measurements [Abstract]
Available-for-sale securities		0	[2]	0	[3]
Recurring Basis [Member] | Quoted Prices in Active Markets (Level 1) [Member] | Debt Securities issued by U.S. Government Agencies [Member]
Fair Value Measurements [Abstract]
Available-for-sale securities	[2]	0		0
Recurring Basis [Member] | Quoted Prices in Active Markets (Level 1) [Member] | Debt Securities issued by the U.S. Treasury [Member]
Fair Value Measurements [Abstract]
Available-for-sale securities	[4]			19,017
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 Measurements [Abstract]
Available-for-sale securities		0	[5]	0	[6]
Recurring Basis [Member] | Significant Other Observable Inputs (Level 2) [Member]
Fair Value Measurements [Abstract]
Cash equivalents	[1]	0		0
Investment in Regulus Therapeutics Inc.		0		0
Total assets		693,199		586,890
Recurring Basis [Member] | Significant Other Observable Inputs (Level 2) [Member] | Corporate Debt Securities [Member]
Fair Value Measurements [Abstract]
Available-for-sale securities		447,597	[2]	372,002	[3]
Recurring Basis [Member] | Significant Other Observable Inputs (Level 2) [Member] | Debt Securities issued by U.S. Government Agencies [Member]
Fair Value Measurements [Abstract]
Available-for-sale securities	[2]	122,282		109,855
Recurring Basis [Member] | Significant Other Observable Inputs (Level 2) [Member] | Debt Securities issued by the U.S. Treasury [Member]
Fair Value Measurements [Abstract]
Available-for-sale securities	[4]			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 Measurements [Abstract]
Available-for-sale securities		123,320	[5]	105,033	[6]
Recurring Basis [Member] | Significant Unobservable Inputs (Level 3) [Member]
Fair Value Measurements [Abstract]
Cash equivalents	[1]	0		0
Investment in Regulus Therapeutics Inc.		0		81,881
Total assets		0		81,881
Recurring Basis [Member] | Significant Unobservable Inputs (Level 3) [Member] | Corporate Debt Securities [Member]
Fair Value Measurements [Abstract]
Available-for-sale securities		0	[2]	0	[3]
Recurring Basis [Member] | Significant Unobservable Inputs (Level 3) [Member] | Debt Securities issued by U.S. Government Agencies [Member]
Fair Value Measurements [Abstract]
Available-for-sale securities	[2]	0		0
Recurring Basis [Member] | Significant Unobservable Inputs (Level 3) [Member] | Debt Securities issued by the U.S. Treasury [Member]
Fair Value Measurements [Abstract]
Available-for-sale securities	[4]			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 Measurements [Abstract]
Available-for-sale securities		$ 0	[5]	$ 0	[6]

[1] Included in cash and cash equivalents on our condensed consolidated balance sheet.

[2] Included in short-term investments on our condensed consolidated balance sheet.

[3] $0.8 million included in cash and cash equivalents on our condensed consolidated balance sheet, with the difference included in short-term investments on our condensed consolidated balance sheet.

[4] $10 million included in cash and cash equivalents on our condensed consolidated balance sheet, with the difference included in short-term investments on our condensed consolidated balance sheet.

[5] $7.6 million included in cash and cash equivalents on our condensed consolidated balance sheet, with the difference included in short-term investments on our condensed consolidated balance sheet.

[6] $9.3 million included in cash and cash equivalents on our condensed consolidated balance sheet, with the difference included in short-term investments on our condensed consolidated balance sheet.