Fair Value Measurements (Details) - USD ($) $ in Thousands		Sep. 30, 2016		Dec. 31, 2015		Sep. 30, 2015
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		630,099		$ 682,728
Investment in Regulus Therapeutics Inc.		$ 9,382		$ 24,792
1 Percent Convertible Senior Notes [Member]
Fair Value Measurements [Abstract]
Interest rate on convertible senior notes		1.00%		1.00%		1.00%
Fair value of convertible notes		$ 467,200
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		$ 137,300
Recurring Basis [Member]
Fair Value Measurements [Abstract]
Cash equivalents	[1]	60,146		$ 88,902
Investment in Regulus Therapeutics Inc.		9,382		24,792
Total assets		690,245		771,630
Recurring Basis [Member] | Corporate Debt Securities [Member]
Fair Value Measurements [Abstract]
Available-for-sale securities	[2]	413,561		438,426
Recurring Basis [Member] | Corporate Debt Securities [Member] | Cash and Cash Equivalents [Member]
Fair Value Measurements [Abstract]
Available-for-sale securities		6,000
Recurring Basis [Member] | Debt Securities issued by U.S. Government Agencies [Member]
Fair Value Measurements [Abstract]
Available-for-sale securities	[2]	63,350		89,253
Recurring Basis [Member] | Debt Securities issued by the U.S. Treasury [Member]
Fair Value Measurements [Abstract]
Available-for-sale securities		28,025	[3]	2,601	[2]
Recurring Basis [Member] | Debt Securities issued by States of the U.S. and Political Subdivisions of the States [Member]
Fair Value Measurements [Abstract]
Available-for-sale securities	[4]	115,781		127,656
Recurring Basis [Member] | Debt Securities issued by States of the U.S. and Political Subdivisions of the States [Member] | Cash and Cash Equivalents [Member]
Fair Value Measurements [Abstract]
Available-for-sale securities		11,100		7,500
Recurring Basis [Member] | Quoted Prices in Active Markets (Level 1) [Member]
Fair Value Measurements [Abstract]
Cash equivalents		60,146		88,902
Investment in Regulus Therapeutics Inc.		9,382		24,792
Total assets		97,553		116,295
Recurring Basis [Member] | Quoted Prices in Active Markets (Level 1) [Member] | Corporate Debt Securities [Member]
Fair Value Measurements [Abstract]
Available-for-sale securities		0		0
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		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		28,025		2,601
Recurring Basis [Member] | Quoted Prices in Active Markets (Level 1) [Member] | Debt Securities issued by States of the U.S. and Political Subdivisions of the States [Member]
Fair Value Measurements [Abstract]
Available-for-sale securities		0		0
Recurring Basis [Member] | Significant Other Observable Inputs (Level 2) [Member]
Fair Value Measurements [Abstract]
Cash equivalents		0		0
Investment in Regulus Therapeutics Inc.		0		0
Total assets		592,692		655,335
Recurring Basis [Member] | Significant Other Observable Inputs (Level 2) [Member] | Corporate Debt Securities [Member]
Fair Value Measurements [Abstract]
Available-for-sale securities		413,561		438,426
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		63,350		89,253
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		0		0
Recurring Basis [Member] | Significant Other Observable Inputs (Level 2) [Member] | Debt Securities issued by States of the U.S. and Political Subdivisions of the States [Member]
Fair Value Measurements [Abstract]
Available-for-sale securities		115,781		127,656
Recurring Basis [Member] | Significant Unobservable Inputs (Level 3) [Member]
Fair Value Measurements [Abstract]
Total assets		$ 0		$ 0

[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] At September 30, 2016, $6.0 million was 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] At September 30, 2016 and December 31, 2015, $11.1 million and $7.5 million, respectively, were 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.