Additional Cash Flow Information (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Cash paid during the year for:
Interest	$ 11,456	$ 11,033	$ 7,668
Income taxes	130,226	129,540	99,754
Non-cash financing activities:
Assets acquired under capital lease obligations	$ 1,252	$ 1,914	$ 3,847