Fair Value Measurements, Assets Measured at Fair Value on a Recurring Basis (Details) - USD ($) $ in Thousands		Mar. 31, 2016	Dec. 31, 2015	Mar. 31, 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		635,737	$ 682,728
Investment in Regulus Therapeutics Inc.		$ 19,703	$ 24,792
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		$ 455,100
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		$ 124,700
Recurring Basis [Member]
Fair Value Measurements [Abstract]
Cash equivalents	[1]	74,091	$ 88,902
Investment in Regulus Therapeutics Inc.		19,703	24,792
Total assets		709,828	771,630
Recurring Basis [Member] | Corporate Debt Securities [Member]
Fair Value Measurements [Abstract]
Available-for-sale securities	[2]	405,152	438,426
Recurring Basis [Member] | Debt Securities issued by U.S. Government Agencies [Member]
Fair Value Measurements [Abstract]
Available-for-sale securities	[2]	79,488	89,253
Recurring Basis [Member] | Debt Securities issued by the U.S. Treasury [Member]
Fair Value Measurements [Abstract]
Available-for-sale securities	[2]	13,101	2,601
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	[3]	118,293	127,656
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		5,600	7,500
Recurring Basis [Member] | Quoted Prices in Active Markets (Level 1) [Member]
Fair Value Measurements [Abstract]
Cash equivalents		74,091	88,902
Investment in Regulus Therapeutics Inc.		19,703	24,792
Total assets		106,895	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		13,101	2,601
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	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		602,933	655,335
Recurring Basis [Member] | Significant Other Observable Inputs (Level 2) [Member] | Corporate Debt Securities [Member]
Fair Value Measurements [Abstract]
Available-for-sale securities		405,152	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		79,488	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 United States and Political Subdivisions of the States [Member]
Fair Value Measurements [Abstract]
Available-for-sale securities		$ 118,293	$ 127,656

[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 March 31, 2016 and December 31, 2015, $5.6 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.