Marketable Securities Marketable Securities - Schedule of Maturities (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Debt Securities, Available-for-sale [Line Items]
Fair Value	$ 70,221	$ 90,590
Corporate bonds
Debt Securities, Available-for-sale [Line Items]
Less than 1 Year	688
1-5 Years	2,775
5-10 Years	0
Greater than 10 Years	0
Fair Value	3,463	6,339
Government securities - U.S.
Debt Securities, Available-for-sale [Line Items]
Less than 1 Year	0
1-5 Years	3,841
5-10 Years	0
Greater than 10 Years	444
Fair Value	4,285	12,701
Other fixed-income securities
Debt Securities, Available-for-sale [Line Items]
Less than 1 Year	50
1-5 Years	3,651
5-10 Years	0
Greater than 10 Years	0
Fair Value	3,701	$ 9,409
Debt Securities, Excluding Money Market Funds, Available For Sale
Debt Securities, Available-for-sale [Line Items]
Less than 1 Year	738
1-5 Years	10,267
5-10 Years	0
Greater than 10 Years	444
Fair Value	$ 11,449

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