Share-Based Compensation (Details) (USD $) In Millions, except Share data, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012
Share-based compensation disclosures
Share Based Compensation expense	$ 6.0	$ 4.3	$ 38.8	$ 8.8
Petrello
Share-based compensation disclosures
Value of incentives in the form of one-time stock grant			18.0
Common stock | Petrello
Share-based compensation disclosures
Value of incentives in the form of one-time stock grant			27.0
Restricted Stock Award
Share-based compensation disclosures
Shares awarded during period			4,688,514	923,131
Shares awarded during period, fair value			76.2	19.2
Fair value of Restricted Stock Awarded to Employees and Directors that vested			36.3	9.3
Restricted Stock Award | Maximum [Member]
Share-based compensation disclosures
Vesting period of shares			4 years
Stock options
Share-based compensation disclosures
Total intrinsic value of stock options exercised			2.4	5.1
Intrinsic value of stock options surrendered				17.9
Total fair value of options vested during the period	$ 3.9	$ 7.6	$ 3.9	$ 7.6
Options awarded (in shares)			44,375	634,974
Fair value of stock options
Weighted-average fair value of options granted (in dollars per share)			$ 6.25	$ 9.52
Weighted-average risk free interest rate (as a percent)			0.64%	0.63%
Dividend yield (as a percent)			0.78%	0.00%
Volatility (as a percent)			51.01%	55.91%
Expected life			4 years	4 years
Stock options | Maximum [Member]
Share-based compensation disclosures
Vesting period of shares			4 years	4 years