Earnings Per Share - Summary of Reconciliation of Numerator and Denominator Used to Calculate Basic and Diluted Loss Per Share of Common Stock (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 14, 2016
Numerator:
Net income	$ 35.4	$ 31.7	$ 39.2	$ 31.4	$ 25.3	$ 32.9	$ 44.1	$ 38.7	$ 137.7	$ 141.0	$ 137.3
Income allocated to participating securities									(1.7)	(2.6)	(2.9)
Net income for basic and diluted EPS									$ 136.0	$ 138.4	$ 134.4
Denominator:
Basic weighted average shares (in shares)									77.4	77.2	74.8
Effect of:
2 1/4% Notes (in shares)									4.5	4.4	1.9
Awards issued under equity plans (in shares)									0.0	0.1	0.1
Diluted weighted average shares (in shares)									81.9	81.7	76.8
Basic:
Basic earnings per share (in USD per share)	$ 0.46	$ 0.40	$ 0.50	$ 0.40	$ 0.32	$ 0.42	$ 0.56	$ 0.49	$ 1.76	$ 1.79	$ 1.80
Diluted:
Diluted earnings per share (in USD per share)	$ 0.43	$ 0.38	$ 0.47	$ 0.37	$ 0.30	$ 0.39	$ 0.54	$ 0.47	$ 1.66	$ 1.69	$ 1.75
2 1/4% Notes
Diluted:
Debt instrument interest rate stated percentage	2.25%				2.25%				2.25%	2.25%		2.25%
2 1/4% Notes | Convertible senior notes
Diluted:
Debt instrument interest rate stated percentage	2.25%								2.25%