Organization and Significant Accounting Policies, Fair Value Measurements on a Recurring Basis (Details) - USD ($) $ in Thousands		Dec. 31, 2018		Dec. 31, 2017
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		1,871,017
Recurring Basis [Member]
Fair Value Measurements [Abstract]
Cash equivalents	[1]	146,281		86,262
Investment in ProQR Therapeutics N.V.	[2]	1,349
Total		2,017,298		994,798
Recurring Basis [Member] | Corporate Debt Securities [Member]
Fair Value Measurements [Abstract]
Available-for-sale securities		1,252,960	[3]	647,461	[4]
Recurring Basis [Member] | Corporate Debt Securities [Member] | Cash and Cash Equivalents [Member]
Fair Value Measurements [Abstract]
Available-for-sale securities		50,200		11,900
Recurring Basis [Member] | Debt Securities issued by U.S. Government Agencies [Member]
Fair Value Measurements [Abstract]
Available-for-sale securities	[5]	276,612		136,325
Recurring Basis [Member] | Debt Securities issued by the U.S. Treasury [Member]
Fair Value Measurements [Abstract]
Available-for-sale securities		260,154	[6]	30,818	[5]
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		14,200
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		79,942	[5]	93,932	[7]
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				3,500
Recurring Basis [Member] | Quoted Prices in Active Markets (Level 1) [Member]
Fair Value Measurements [Abstract]
Cash equivalents		146,281		86,262
Investment in ProQR Therapeutics N.V.		0
Total		406,435		117,080
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		260,154		30,818
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 ProQR Therapeutics N.V.		0
Total		1,609,514		877,718
Recurring Basis [Member] | Significant Other Observable Inputs (Level 2) [Member] | Corporate Debt Securities [Member]
Fair Value Measurements [Abstract]
Available-for-sale securities		1,252,960		647,461
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		276,612		136,325
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		79,942		93,932
Recurring Basis [Member] | Significant Unobservable Inputs (Level 3) [Member]
Fair Value Measurements [Abstract]
Cash equivalents		0
Investment in ProQR Therapeutics N.V.		1,349
Total		1,349		$ 0
Recurring Basis [Member] | Significant Unobservable Inputs (Level 3) [Member] | Corporate Debt Securities [Member]
Fair Value Measurements [Abstract]
Available-for-sale securities		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
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
Recurring Basis [Member] | Significant Unobservable Inputs (Level 3) [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

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

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

[3] $50.2 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] $11.9 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] Included in short-term investments on our consolidated balance sheet.

[6] $14.2 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.

[7] $3.5 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.