Fair Value of Financial Instruments - Carrying Value (Details) - Recurring - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Total Fair Value
U.S. government agency debt securities
Other noncurrent assets	$ 418	$ 326
Total assets at fair value	939,264	775,142
Quoted Prices in Active Markets for Identical Assets (Level 1)
U.S. government agency debt securities
Other noncurrent assets	3	1
Total assets at fair value	181,638	192,939
Significant Other Observable Inputs (Level 2)
U.S. government agency debt securities
Other noncurrent assets	415	325
Total assets at fair value	757,626	582,203
Cash \| Total Fair Value
Assets:
Cash and cash equivalents	78,912	106,918
Cash \| Quoted Prices in Active Markets for Identical Assets (Level 1)
Assets:
Cash and cash equivalents	78,912	106,918
Cash \| Significant Other Observable Inputs (Level 2)
Assets:
Cash and cash equivalents	0	0
Money market funds \| Total Fair Value
Assets:
Cash and cash equivalents	102,469	85,330
Money market funds \| Quoted Prices in Active Markets for Identical Assets (Level 1)
Assets:
Cash and cash equivalents	102,469	85,330
Money market funds \| Significant Other Observable Inputs (Level 2)
Assets:
Cash and cash equivalents	0	0
Corporate debt securities \| Total Fair Value
Assets:
Short-term marketable securities	165,527	163,770
Municipal debt securities
Long-term marketable securities	571,828	415,650
Corporate debt securities \| Quoted Prices in Active Markets for Identical Assets (Level 1)
Assets:
Short-term marketable securities	0	245
Municipal debt securities
Long-term marketable securities	254	445
Corporate debt securities \| Significant Other Observable Inputs (Level 2)
Assets:
Short-term marketable securities	165,527	163,525
Municipal debt securities
Long-term marketable securities	571,574	415,205
Municipal debt securities \| Total Fair Value
Assets:
Short-term marketable securities	165
Municipal debt securities
Long-term marketable securities		165
Municipal debt securities \| Quoted Prices in Active Markets for Identical Assets (Level 1)
Assets:
Short-term marketable securities	0
Municipal debt securities
Long-term marketable securities		0
Municipal debt securities \| Significant Other Observable Inputs (Level 2)
Assets:
Short-term marketable securities	165
Municipal debt securities
Long-term marketable securities		165
U.S. government agency debt securities \| Total Fair Value
Municipal debt securities
Long-term marketable securities	19,945	2,983
U.S. government agency debt securities \| Quoted Prices in Active Markets for Identical Assets (Level 1)
Municipal debt securities
Long-term marketable securities	0	0
U.S. government agency debt securities \| Significant Other Observable Inputs (Level 2)
Municipal debt securities
Long-term marketable securities	$ 19,945	$ 2,983

Fair value portion of probable future economic benefits obtained or controlled by an entity as a result of past transactions or events.

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