Stock Plans (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Defined Benefit Plan Disclosure [Line Items]
Contribution expense	$ 6,921	$ 6,644	$ 5,907
Retirement Savings Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Employer match of employee contributions to Retirement Savings Plan	50.00%
Share-based compensation arrangement by share-based payment award, maximum salary percentage of employer match	6.00%
Contribution expense	$ 4,333	$ 4,088	$ 3,660
Shares held by Retirement Savings Plan	1,075,692