Fair Value of Financial Instruments - Carrying Value (Details) - Recurring - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019
Quoted Prices in Active Markets for Identical Assets (Level 1)
Other noncurrent assets
Marketable securities - restricted (SERP)	$ 2	$ 3
Total assets at fair value	204,553	181,638
Significant Other Observable Inputs (Level 2)
Other noncurrent assets
Marketable securities - restricted (SERP)	462	415
Total assets at fair value	536,046	757,626
Total Fair Value
Other noncurrent assets
Marketable securities - restricted (SERP)	464	418
Total assets at fair value	740,599	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	258	254
Corporate debt securities \| Significant Other Observable Inputs (Level 2)
Marketable securities
Marketable securities	147,657	165,527
Long term marketable securities
Long term marketable securities	387,927	571,574
Corporate debt securities \| Total Fair Value
Marketable securities
Marketable securities	147,657	165,527
Long term marketable securities
Long term marketable securities	388,185	571,828
Municipal debt securities \| Quoted Prices in Active Markets for Identical Assets (Level 1)
Marketable securities
Marketable securities		0
Municipal debt securities \| Significant Other Observable Inputs (Level 2)
Marketable securities
Marketable securities		165
Municipal debt securities \| Total Fair Value
Marketable securities
Marketable securities		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
U.S. government agency and municipal debt securities \| Significant Other Observable Inputs (Level 2)
Long term marketable securities
Long term marketable securities		19,945
U.S. government agency and municipal debt securities \| Total Fair Value
Long term marketable securities
Long term marketable securities		19,945
Cash \| Quoted Prices in Active Markets for Identical Assets (Level 1)
Assets:
Cash and cash equivalents	188,974	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	188,974	78,912
Money market funds \| Quoted Prices in Active Markets for Identical Assets (Level 1)
Assets:
Cash and cash equivalents	15,319	102,469
Money market funds \| Significant Other Observable Inputs (Level 2)
Assets:
Cash and cash equivalents	0	0
Money market funds \| Total Fair Value
Assets:
Cash and cash equivalents	$ 15,319	$ 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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Amount of asset, recognized in statement of financial position, for overfunded defined benefit pension and other postretirement plans.

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