Supplemental Cash Flow Information (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Cash paid:
Income taxes	$ 387,159	$ 339,411	$ 156,075
Interest	424,547	406,987	405,120
Non-cash investing and financing activities:
Fixed assets under capital lease obligations	$ 48,378	$ 28,127	$ 74,035