Other Long-Term Liabilities- New Jersey tax benefits (Details) - USD ($) $ in Thousands	1 Months Ended				3 Months Ended		6 Months Ended
	Nov. 30, 2015	Dec. 31, 2014	Jan. 31, 2014	Jan. 31, 2013	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Investment and other income					$ 466	$ 362	$ 1,245	$ 1,213
New Jersey tax benefits
Amount of tax benefit	$ 9,800	$ 1,900	$ 1,100	$ 800
Proceeds from sale of tax credits	$ 9,200	$ 1,800	$ 1,000	$ 800
Base of operations requirement (in years)							5 years
Investment and other income					$ 0	$ 0	$ 400	$ 500