Shareholders' Equity and Earnings Per Common Share - Calculation of Basic and Diluted Per Share (Details) (USD $) In Millions, except Per Share data, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013
Equity [Abstract]
Net income (loss)	$ 412.9	$ (85.3)	$ 684.5	$ (310.8)
Less: Net income (loss) attributable to noncontrolling interest	166.6	(3.1)	180.1	(16.6)
Net income (loss) available to common shareholders	$ 246.3	$ (82.2)	$ 504.4	$ (294.2)
Weighted-average common shares outstanding, Basic:
Basic	254.5	250.3	257.8	240.2
Weighted-average common shares outstanding, Dilutive Effects:
Antidilutive Securities Excluded from Computation of Earnings Per Share, Amount		0.5		0.3
Dilutive	256.2	250.3	259.8	240.2
Net income (loss) per common share
Basic	$ 0.97	$ (0.33)	$ 1.96	$ (1.22)
Diluted	$ 0.96	$ (0.33)	$ 1.94	$ (1.22)
Restricted Stock Units (RSUs)
Weighted-average common shares outstanding, Dilutive Effects:
Dilutive effects of share-based payments	1.1	0	1.0	0
Performance shares (PSU awards)
Weighted-average common shares outstanding, Dilutive Effects:
Dilutive effects of share-based payments	0	0	0.3	0
RSUs - Deal incentive awards
Weighted-average common shares outstanding, Dilutive Effects:
Dilutive effects of share-based payments	0.6	0	0.7	0