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Marketable Securities (Details) - USD ($) $ in Thousands	12 Months Ended
Marketable Securities (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Jan. 01, 2018
Debt Securities, Available-for-sale [Line Items]
Reclassification pursuant to change in accounting principle		$ 0
Proceeds from sale of common stock	$ 23,933	0	$ 0
Recognized loss on sale of common stock included in interest and other income	5,000
Cost or Amortized Cost	249,944	276,567
Unrealized Loss	(342)
Unrealized Gain, Net		361
Marketable securities	$ 249,602	276,928
Accounting Standards Update 2016-01 \| Cumulative Other Comprehensive Loss
Debt Securities, Available-for-sale [Line Items]
Reclassification pursuant to change in accounting principle		(1,902)		$ (1,900)
Accounting Standards Update 2016-01 \| Retained Earnings
Debt Securities, Available-for-sale [Line Items]
Reclassification pursuant to change in accounting principle		$ 1,902		$ 1,900