Fair Value of Financial Instruments - Carrying Value (Details) - Recurring - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Quoted Prices in Active Markets for Identical Assets (Level 1)
Assets:
Cash and cash equivalents	$ 116,889	$ 192,248
Other noncurrent assets
Marketable securities - restricted (SERP)	1	1
Total assets at fair value	117,593	192,939
Significant Other Observable Inputs (Level 2)
Assets:
Cash and cash equivalents	0	0
Other noncurrent assets
Marketable securities - restricted (SERP)	377	325
Total assets at fair value	775,924	582,203
Total Fair Value
Assets:
Cash and cash equivalents	116,889	192,248
Other noncurrent assets
Marketable securities - restricted (SERP)	378	326
Total assets at fair value	893,517	775,142
Corporate debt securities \| Quoted Prices in Active Markets for Identical Assets (Level 1)
Marketable securities
Marketable securities	247	245
Long term marketable securities
Long term marketable securities	456	445
Corporate debt securities \| Significant Other Observable Inputs (Level 2)
Marketable securities
Marketable securities	179,396	163,525
Long term marketable securities
Long term marketable securities	584,990	415,205
Corporate debt securities \| Total Fair Value
Marketable securities
Marketable securities	179,643	163,770
Long term marketable securities
Long term marketable securities	585,446	415,650
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	0
U.S. government agency and municipal debt securities \| Significant Other Observable Inputs (Level 2)
Long term marketable securities
Long term marketable securities	10,996	3,148
U.S. government agency and municipal debt securities \| Total Fair Value
Long term marketable securities
Long term marketable securities	$ 10,996	$ 3,148

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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