Income Taxes (Narrative) (Details) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Tax Disclosure [Abstract]
Reduction of net deferred tax liability	$ 38,400
Tax benefits associated with the exercise, vesting or disposition of stock under stock plans recognized as an adjustment of additional paid-in capital		$ 8,300	$ 11,800	$ 5,000
Income taxes paid		126,000	144,100	$ 138,000
Provisional estimate for transition tax		1,200
Unrecognized tax benefits accrued interest	$ 433	$ 433	$ 277