Other Long-Term Liabilities- New Jersey tax benefits (Details) - USD ($) $ in Thousands	1 Months Ended			12 Months Ended
	Nov. 30, 2015	Dec. 31, 2014	Jan. 31, 2014	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Investment and other income				$ 4,487	$ 4,214	$ 4,386
New Jersey tax benefits
Amount of tax benefit	$ 9,800	$ 1,900	$ 1,100
Proceeds from sale of tax credits	$ 9,200	$ 1,800	$ 1,000
Base of operations requirement (in years)				5 years
Investment and other income				$ 2,500	$ 2,700	$ 2,800