Earnings Per Share (Comparison Of Basic And Diluted Earnings Per Share) (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Basic earnings per share:
Net income	$ (40.8)	$ 68.6	$ 153.4	$ 74.1	$ 1.8	$ (51.7)	$ 9.5	$ 124.3	$ 255.3	$ 83.9	$ 490.3
Weighted average common shares outstanding	87,036,339	87,102,290	88,742,484	90,254,512	90,934,107	90,882,511	92,441,730	95,935,551	88,275,810	92,530,208	97,538,319
Basic earnings per share	$ (0.47)	$ 0.79	$ 1.73	$ 0.82	$ 0.02	$ (0.57)	$ 0.10	$ 1.30	$ 2.89	$ 0.91	$ 5.03
Diluted earnings per share:
Net income	$ (40.8)	$ 68.6	$ 153.4	$ 74.1	$ 1.8	$ (51.7)	$ 9.5	$ 124.3	$ 255.3	$ 83.9	$ 490.3
Weighted average common shares outstanding	87,036,339	87,102,290	88,742,484	90,254,512	90,934,107	90,882,511	92,441,730	95,935,551	88,275,810	92,530,208	97,538,319
Share equivalents:
Warrants and options									839,038	1,031,666	1,432,960
RSUs and LTIP awards									677,033	591,773	605,808
Employee share purchase plan									9,013	20,813	14,686
Weighted average common shares and common share equivalents outstanding - diluted	87,036,339	88,603,101	90,040,509	91,559,225	92,422,422	90,882,511	93,984,226	97,577,029	89,800,894	94,174,460	99,591,773
Diluted earnings per share	$ (0.47)	$ 0.77	$ 1.70	$ 0.81	$ 0.02	$ (0.57)	$ 0.10	$ 1.27	$ 2.84	$ 0.89	$ 4.92