Marketable Securities Marketable Securities - Schedule of Maturities (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Schedule of Available-for-sale Securities [Line Items]
Fair Value	$ 157,269	$ 77,511
Corporate bonds
Schedule of Available-for-sale Securities [Line Items]
Less than 1 Year	18,502
1-5 Years	22,812
5-10 Years	0
Greater than 10 Years	0
Fair Value	41,314	9,262
Government securities - U.S.
Schedule of Available-for-sale Securities [Line Items]
Less than 1 Year	0
1-5 Years	36,271
5-10 Years	506
Greater than 10 Years	0
Fair Value	36,777	19,050
Other fixed-income securities
Schedule of Available-for-sale Securities [Line Items]
Less than 1 Year	1,207
1-5 Years	7,749
5-10 Years	0
Greater than 10 Years	0
Fair Value	8,956	$ 4,250
Total debt securities
Schedule of Available-for-sale Securities [Line Items]
Less than 1 Year	19,709
1-5 Years	66,832
5-10 Years	506
Greater than 10 Years	0
Fair Value	$ 87,047