Earnings per Share - Calculation of Basic and Diluted EPS (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Calculation of basic EPS:
Net income applicable to common stock									$ 43.9	$ 93.5	$ 473.4
Weighted-average common shares outstanding (in shares)									115.7	97.1	90.9
Basic EPS (in dollars per share)	$ (0.80)	$ 0.38	$ 0.44	$ 0.35	$ (0.37)	$ 0.39	$ 0.59	$ 0.50	$ 0.38	$ 0.96	$ 5.21
Calculation of diluted EPS:
Diluted weighted-average common shares outstanding (in shares)									117.8	100.3	93.4
Diluted EPS (in dollars per share)	$ (0.80)	$ 0.38	$ 0.44	$ 0.35	$ (0.37)	$ 0.38	$ 0.57	$ 0.48	$ 0.37	$ 0.93	$ 5.07