Supplemental Gas and Oil Information (Unaudited) Standardized Measure Of Future Net Cash Flows (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Discounted Future Net Cash Flows Relating to Proved Oil and Gas Reserves [Line Items]
Future cash inflows	$ 22,028.9	$ 16,239.8	$ 15,325.3
Future production costs	(9,074.2)	(7,789.0)	(7,389.9)
Future development costs	(4,726.0)	(3,432.9)	(2,202.5)
Future income tax expenses	(1,439.1)	(913.4)	(1,169.3)
Future net cash flows	6,789.6	4,104.5	4,563.6
10% annual discount for estimated timing of net cash flows	(3,692.3)	(2,176.5)	(2,087.3)
Standardized measure of discounted future net cash flows	$ 3,097.3	$ 1,928.0	$ 2,476.3	$ 5,340.0