Basic and Diluted Earnings Per Share (Detail) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Numerator
Net Income	$ 248.0	$ 236.0	$ 236.8	$ 210.6	$ 226.1	$ 203.8	$ 224.3	$ 212.9	$ 931.4	$ 867.1	$ 402.1
Denominator
Weighted Average Common Shares - Basic									235,445,700	246,986,700	255,525,900
Dilution for Assumed Exercises of Stock Options and Nonvested Stock Awards									533,500	868,000	1,126,900
Weighted Average Common Shares - Assuming Dilution									235,979,200	247,854,700	256,652,800
Net Income Per Common Share
Basic	$ 1.07	$ 1.01	$ 1.00	$ 0.88	$ 0.93	$ 0.83	$ 0.90	$ 0.85	$ 3.96	$ 3.51	$ 1.57
Assuming Dilution	$ 1.07	$ 1.01	$ 1.00	$ 0.88	$ 0.93	$ 0.83	$ 0.90	$ 0.84	$ 3.95	$ 3.50	$ 1.57