Quarterly Data - Schedule of Financial Data by Quarter (Detail) - USD ($) $ / shares in Units, $ 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
Quarterly Financial Information Disclosure [Abstract]
Revenues	$ 676.1	$ 693.8	$ 680.6	$ 693.0	$ 695.5	$ 687.2	$ 605.7	$ 578.0	$ 2,743.5	$ 2,566.4	$ 2,355.1
Income from operations	126.0	146.6	143.3	146.2	112.1	145.7	132.0	115.6	562.1	505.4	454.6
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
Net income	$ 50.5	$ 53.2	$ 11.8	$ 58.0	$ 10.8	$ 11.1	$ 58.4	$ 50.1	$ 173.5	$ 130.4	$ 197.7
EPS—Basic
Earnings per share from continuing operations - Basic	$ 0.22	$ 0.28	$ 0.26	$ 0.27	$ 0.11	$ 0.32	$ 0.27	$ 0.22	$ 1.03	$ 0.92	$ 0.88
Net income	0.21	0.23	0.05	0.25	0.04	0.04	0.27	0.23	0.74	0.58	0.92
EPS—Diluted
Earnings per share from continuing operations - Diluted	0.22	0.28	0.26	0.27	0.11	0.32	0.27	0.22	1.03	0.92	0.88
Net income	0.21	0.23	0.05	0.25	0.04	0.04	0.27	0.23	0.74	0.58	0.92
Dividends paid per common share outstanding	$ 0.225	$ 0.225	$ 0.225	$ 0.225	$ 0.220	$ 0.220	$ 0.220	$ 0.220	$ 0.90	$ 0.88	$ 0.88