Stock Plans (Details) (USD $) In Thousands, except Share data, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Defined Benefit Plan Disclosure [Line Items]
Contribution expense	$ 5,319	$ 5,106	$ 3,717
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	$ 3,504	$ 3,399	$ 2,481
Shares held by Retirement Savings Plan	3,163,585