Stock Plans (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Defined Benefit Plan Disclosure [Line Items]
Contribution expense	$ 5,907	$ 5,347	$ 5,213
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%
Annual Retirement Contribution Percent	4.00%
Contribution expense	$ 3,660	$ 3,666	$ 3,278
Shares held by Retirement Savings Plan	1,226,034