Net Income (Loss) Per Common Share (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Earnings Per Share, Diluted, by Common Class, Including Two Class Method [Line Items]
Weighted Average Number of Shares Outstanding, Basic	504.2	462.3	404.9	391.3	391.3	364.7	346.4	331.0	441.1	358.6	331.9
Dilutive Securities, Effect on Basic Earnings Per Share, Options and Restrictive Stock Units									$ 0.3	$ 0.1	$ 0.0
Weighted Average Number of Shares Outstanding, Diluted	504.2	462.7	405.2	391.5	391.5	364.9	346.5	331.1	441.4	358.7	331.9
Net Income (Loss) Available to Common Stockholders, Basic	$ (813.0)	$ 207.0	$ 284.0	$ 415.0	$ 376.0	$ 271.0	$ 17.0	$ 69.0	$ 93.0	$ 733.0	$ 595.0
Earnings Per Share, Basic	$ (1.61)	$ 0.45	$ 0.70	$ 1.06	$ 0.96	$ 0.74	$ 0.05	$ 0.21	$ 0.21	$ 2.04	$ 1.79
Earnings Per Share, Diluted									$ 0.21	$ 2.04	$ 1.79