Earnings Per Share - Schedule of Earnings Per Share (Detail) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Basic earnings per share
Net income from continuing operations	$ 51.0	$ 64.9	$ 61.5	$ 63.8	$ 27.4	$ 73.0	$ 57.6	$ 48.4	$ 241.2	$ 206.4	$ 188.7
Amount allocated to nonvested participating shareholders									(0.7)	(0.7)	(0.6)
Income before discontinued operations available to common shareholders—Basic									240.5	205.7	188.1
Income (loss) from discontinued operations									(67.7)	(76.0)	9.0
Amount allocated to nonvested participating shareholders									0.0	0.0	0.0
Income (loss) from discontinued operations —Basic									(67.7)	(76.0)	9.0
Net income	$ 50.5	$ 53.2	$ 11.8	$ 58.0	$ 10.8	$ 11.1	$ 58.4	$ 50.1	173.5	130.4	197.7
Amount allocated to nonvested participating shareholders									(0.7)	(0.7)	(0.6)
Net income available to common shareholders—Basic									$ 172.8	$ 129.7	$ 197.1
Average common shares outstanding - Basic									233.1	223.1	215.0
Earnings per share from continuing operations available to common shareholders—Basic	$ 0.22	$ 0.28	$ 0.26	$ 0.27	$ 0.11	$ 0.32	$ 0.27	$ 0.22	$ 1.03	$ 0.92	$ 0.88
Earnings per share from discontinued operations available to common shareholders—Basic									(0.29)	(0.34)	0.04
Earnings per share - Basic	$ 0.21	$ 0.23	$ 0.05	$ 0.25	$ 0.04	$ 0.04	$ 0.27	$ 0.23	$ 0.74	$ 0.58	$ 0.92
Diluted earnings per share
Net income from continuing operations	$ 51.0	$ 64.9	$ 61.5	$ 63.8	$ 27.4	$ 73.0	$ 57.6	$ 48.4	$ 241.2	$ 206.4	$ 188.7
Amount allocated to nonvested participating shareholders									(0.7)	(0.7)	(0.6)
Income before discontinued operations available to common shareholders—Diluted									240.5	205.7	188.1
Income (loss) from discontinued operations									(67.7)	(76.0)	9.0
Amount allocated to nonvested participating shareholders									0.0	0.0	0.0
Income (loss) from discontinued operations available to common shareholders—Diluted									(67.7)	(76.0)	9.0
Net income	$ 50.5	$ 53.2	$ 11.8	$ 58.0	$ 10.8	$ 11.1	$ 58.4	$ 50.1	173.5	130.4	197.7
Amount allocated to nonvested participating shareholders									(0.7)	(0.7)	(0.6)
Net income available to common shareholders—Diluted									$ 172.8	$ 129.7	$ 197.1
Average common shares outstanding - Basic									233.1	223.1	215.0
Assumed conversion of stock options, unvested restricted stock and contingent performance shares, net									1.4	0.6	0.5
Average common shares outstanding—Diluted									234.5	223.7	215.5
Earnings per share from continuing operations available to common shareholders—Diluted	$ 0.22	$ 0.28	$ 0.26	$ 0.27	$ 0.11	$ 0.32	$ 0.27	$ 0.22	$ 1.03	$ 0.92	$ 0.88
Earnings per share from discontinued operations available to common shareholders—Diluted									(0.29)	(0.34)	0.04
Earnings per share - Diluted	$ 0.21	$ 0.23	$ 0.05	$ 0.25	$ 0.04	$ 0.04	$ 0.27	$ 0.23	$ 0.74	$ 0.58	$ 0.92
Anti-dilutive shares									0.0	0.0	0.0