Schedule of Computation of Basic and Diluted EPS under Two-class Method (Detail) - 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. 30, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Basic and diluted:
Net income from continuing operations										$ 186.9	$ 461.5	$ 375.9
Net loss from discontinued operations												(185.5)
Net income attributable to controlling interest		$ 188.5	$ 98.3	$ (174.7)	$ 74.8	$ 155.6	$ 85.4	$ 109.1	$ 111.4	186.9	461.5	190.4
Participating share awards with dividend equivalent rights										(0.0)	(0.0)	(0.0)
Net income available to common shareholders										186.9	461.5	190.4
Earnings allocated to participating share awards	[1]									(0.0)	(0.0)	(0.0)
Net income attributable to common shareholders										$ 186.9	$ 461.5	$ 190.4
Denominator:
Basic: Weighted average common stock	[1]									87.3	87.2	87.1
Add: Weighted average stock options/share awards	[1]									0.2	0.2	0.2
Diluted:	[1]									87.5	87.4	87.3
Basic EPS:
Continuing operations	[2]									$ 2.14	$ 5.29	$ 4.32
Discontinued operations	[2]											(2.13)
Basic earnings per share		$ 2.16	$ 1.13	$ (2.00)	$ 0.86	$ 1.78	$ 0.98	$ 1.25	$ 1.28	2.14	5.29	2.19
Diluted EPS:
Continuing operations	[2]									2.14	5.29	4.31
Discontinued operations	[2]											(2.13)
Diluted earnings per share		$ 2.15	$ 1.12	$ (2.00)	$ 0.86	$ 1.78	$ 0.98	$ 1.25	$ 1.27	$ 2.14	$ 5.29	$ 2.18

[1] The Company’s unvested RSUs and PSs, of which some included the right to receive non-forfeitable dividend equivalents, are considered participating securities. Calculations of EPS under the two-class method exclude from the numerator any dividends paid or owed on participating securities and any undistributed earnings considered to be attributable to participating securities. The related participating securities are similarly excluded from the denominator.

[2] Participating share awards with the right to receive dividend equivalents are (under the two class method) excluded from the earnings per share calculation (see Note 22 in this Annual Report).