Marketable Securities Marketable Securities - Schedule of Maturities (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Schedule of Available-for-sale Securities [Line Items]
Fair Value	$ 134,629	$ 77,511
Corporate bonds
Schedule of Available-for-sale Securities [Line Items]
Less than 1 Year	20,673
1-5 Years	14,365
5-10 Years	0
Greater than 10 Years	0
Fair Value	35,038	9,262
Government securities - U.S.
Schedule of Available-for-sale Securities [Line Items]
Less than 1 Year	0
1-5 Years	13,909
5-10 Years	0
Greater than 10 Years	990
Fair Value	14,899	19,050
Other fixed-income securities
Schedule of Available-for-sale Securities [Line Items]
Less than 1 Year	51
1-5 Years	7,899
5-10 Years	0
Greater than 10 Years	0
Fair Value	7,950	$ 4,250
Total debt securities
Schedule of Available-for-sale Securities [Line Items]
Less than 1 Year	20,724
1-5 Years	36,173
5-10 Years	0
Greater than 10 Years	990
Fair Value	$ 57,887