Statement of Cash Flows (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017	Dec. 31, 2016
Supplemental Cash Flow Information [Abstract]
Cash and cash equivalents	$ 1,019,491	$ 811,773	$ 812,733
Restricted Cash	12,500	12,500
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents	1,031,991	824,273	825,233	$ 547,703
Accrued capital expenditures	40,200	32,300	$ 41,800	$ 28,200
Capital Lease Obligations Incurred	$ 0	$ 201,200