Basic and Diluted Earnings Per Share (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
Numerator
Net Income	$ 226.1	$ 203.8	$ 224.3	$ 212.9	$ (282.2)	$ 219.1	$ 239.4	$ 225.8	$ 867.1	$ 402.1	$ 847.0
Denominator
Weighted Average Common Shares - Basic									246,986,700	255,525,900	264,725,800
Dilution for Assumed Exercises of Stock Options and Nonvested Stock Awards									868,000	1,126,900	1,223,400
Weighted Average Common Shares - Assuming Dilution									247,854,700	256,652,800	265,949,200
Net Income Per Common Share
Basic	$ 0.93	$ 0.83	$ 0.90	$ 0.85	$ (1.12)	$ 0.86	$ 0.93	$ 0.87	$ 3.51	$ 1.57	$ 3.20
Assuming Dilution	$ 0.93	$ 0.83	$ 0.90	$ 0.84	$ (1.12)	$ 0.86	$ 0.93	$ 0.86	$ 3.50	$ 1.57	$ 3.19