Stock-based Compensation Stock option exercise (Details) - USD ($) $ / shares in Units, $ in Thousands		3 Months Ended
		Mar. 31, 2018	Mar. 31, 2017
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]
Number of stock options exercised		209,420	94,730
Intrinsic value of stock options exercised	[1]	$ 5,845	$ 994
Cash received upon exercise of stock options		4,090	2,094
Income tax benefit		$ 1,288	$ 381
Share Price		$ 46.01	$ 31.35

[1] The intrinsic value of stock options exercised was computed using the closing share price on March 29, 2018 of $46.01 and on March 31, 2017 of $31.35, as applicable.