Supplemental Gas and Oil Information (Unaudited) Standardized Measure Of Future Net Cash Flows (Details) - USD ($) $ in Millions	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Standardized Measure of Discounted Future Net Cash Flows Relating to Proved Oil and Gas Reserves [Abstract]
Future cash inflows	$ 16,239.8	$ 15,325.3	$ 28,167.3
Future production costs	(7,789.0)	(7,389.9)	(9,842.1)
Future development costs	(3,432.9)	(2,202.5)	(3,521.3)
Future income tax expenses	(913.4)	(1,169.3)	(4,304.0)
Future net cash flows	4,104.5	4,563.6	10,499.9
10% annual discount for estimated timing of net cash flows	(2,176.5)	(2,087.3)	(5,159.9)
Standardized measure of discounted future net cash flows	$ 1,928.0	$ 2,476.3	$ 5,340.0	$ 4,383.9