Fair Value of Financial Instruments - Carrying Value (Details) - Recurring - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Quoted Prices in Active Markets for Identical Assets (Level 1)
Other noncurrent assets
Marketable securities - restricted (SERP)	$ 8	$ 3
Total assets at fair value	226,030	181,638
Significant Other Observable Inputs (Level 2)
Other noncurrent assets
Marketable securities - restricted (SERP)	334	415
Total assets at fair value	709,895	757,626
Total Fair Value
Other noncurrent assets
Marketable securities - restricted (SERP)	342	418
Total assets at fair value	935,925	939,264
Corporate debt securities \| Quoted Prices in Active Markets for Identical Assets (Level 1)
Marketable securities
Marketable securities	0	0
Long term marketable securities
Long term marketable securities	255	254
Corporate debt securities \| Significant Other Observable Inputs (Level 2)
Marketable securities
Marketable securities	174,939	165,527
Long term marketable securities
Long term marketable securities	524,428	571,574
Corporate debt securities \| Total Fair Value
Marketable securities
Marketable securities	174,939	165,527
Long term marketable securities
Long term marketable securities	524,683	571,828
Municipal debt securities \| Quoted Prices in Active Markets for Identical Assets (Level 1)
Marketable securities
Marketable securities	0	0
Municipal debt securities \| Significant Other Observable Inputs (Level 2)
Marketable securities
Marketable securities	165	165
Municipal debt securities \| Total Fair Value
Marketable securities
Marketable securities	165	165
U.S. government agency and municipal debt securities \| Quoted Prices in Active Markets for Identical Assets (Level 1)
Long term marketable securities
Long term marketable securities	0	0
U.S. government agency and municipal debt securities \| Significant Other Observable Inputs (Level 2)
Long term marketable securities
Long term marketable securities	10,029	19,945
U.S. government agency and municipal debt securities \| Total Fair Value
Long term marketable securities
Long term marketable securities	10,029	19,945
Cash \| Quoted Prices in Active Markets for Identical Assets (Level 1)
Assets:
Cash and cash equivalents	19,911	78,912
Cash \| Significant Other Observable Inputs (Level 2)
Assets:
Cash and cash equivalents	0	0
Cash \| Total Fair Value
Assets:
Cash and cash equivalents	19,911	78,912
Money market funds \| Quoted Prices in Active Markets for Identical Assets (Level 1)
Assets:
Cash and cash equivalents	205,856	102,469
Money market funds \| Significant Other Observable Inputs (Level 2)
Assets:
Cash and cash equivalents		0
Money market funds \| Total Fair Value
Assets:
Cash and cash equivalents	$ 205,856	$ 102,469

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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