Organization and Significant Accounting Policies, Fair Value Measurements on a Recurring Basis (Details) - USD ($) $ in Thousands		Dec. 31, 2015		Dec. 31, 2014
Organization and Significant Accounting Policies [Abstract]
Transfers from Level 1 to Level 2		$ 0		$ 0
Transfers from Level 2 to Level 1		0		0
Fair Value Measurements [Abstract]
Available-for-sale securities		682,728		687,788
Investment in Regulus Therapeutics Inc.		24,792		81,881
Recurring Basis [Member]
Fair Value Measurements [Abstract]
Cash equivalents	[1]	88,902		104,680
Investment in Regulus Therapeutics Inc.		24,792		81,881
Total assets		771,630		792,468
Recurring Basis [Member] | Corporate Debt Securities [Member]
Fair Value Measurements [Abstract]
Available-for-sale securities		438,426	[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]	89,253		109,855
Recurring Basis [Member] | Debt Securities issued by the U.S. Treasury [Member]
Fair Value Measurements [Abstract]
Available-for-sale securities		2,601	[2]	19,017	[4]
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		127,656	[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		7,500		9,300
Recurring Basis [Member] | Quoted Prices in Active Markets (Level 1) [Member]
Fair Value Measurements [Abstract]
Cash equivalents		88,902		104,680
Investment in Regulus Therapeutics Inc.		24,792		0
Total assets		116,295		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		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		2,601		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		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		655,335		586,890
Recurring Basis [Member] | Significant Other Observable Inputs (Level 2) [Member] | Corporate Debt Securities [Member]
Fair Value Measurements [Abstract]
Available-for-sale securities		438,426		372,002
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		89,253		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		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		127,656		105,033
Recurring Basis [Member] | Significant Unobservable Inputs (Level 3) [Member]
Fair Value Measurements [Abstract]
Cash equivalents		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		0
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		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		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 Measurements [Abstract]
Available-for-sale securities		$ 0		$ 0

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

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

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

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

[5] $7.5 million included in cash and cash equivalents on our consolidated balance sheet, with the difference included in short-term investments on our consolidated balance sheet.

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