Earnings per Share (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Determination of shares:
Weighted average common shares outstanding (in shares)	115,200,000	91,900,000	115,800,000	91,500,000
Diluted weighted average common shares outstanding (in shares)	116,700,000	95,300,000	117,700,000	95,000,000.0
Anti-dilutive securities, stock options (in shares)	2,519,816	660,436	2,139,874	655,952
Calculation of basic EPS:
Net income applicable to common stock	$ 43.9	$ 36.1	$ 136.4	$ 135.5
Weighted average common shares outstanding (in shares)	115,200,000	91,900,000	115,800,000	91,500,000
Basic EPS (in dollars per share)	$ 0.38	$ 0.39	$ 1.18	$ 1.48
Calculation of diluted EPS using two class method:
Net income applicable to common stock	$ 43.9	$ 36.1	$ 136.4	$ 135.5
Diluted weighted average common shares outstanding (in shares)	116,700,000	95,300,000	117,700,000	95,000,000.0
Diluted EPS (in dollars per share)	$ 0.38	$ 0.38	$ 1.16	$ 1.43
Stock options
Determination of shares:
Assumed conversion of dilutive employee stock-based awards and restricted stock (in shares)	1,400,000	3,200,000	1,700,000	3,300,000
Restricted stock
Determination of shares:
Assumed conversion of dilutive employee stock-based awards and restricted stock (in shares)	100,000	200,000	200,000	200,000