Basic and Diluted Earnings Per Share (Detail) (USD $) In Millions, except Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Numerator
Net Income	$ (279.1)	$ 221.1	$ 242.5	$ 228.9	$ 221.2	$ 205.7	$ 218.6	$ 212.6	$ 413.4	$ 858.1	$ 894.4
Denominator
Weighted Average Common Shares - Basic									255,525,900	264,725,800	281,355,900
Dilution for Assumed Exercises of Stock Options and Nonvested Stock Awards									1,126,900	1,223,400	400,900
Weighted Average Common Shares - Assuming Dilution									256,652,800	265,949,200	281,756,800
Net Income Per Common Share
Basic	$ (1.11)	$ 0.87	$ 0.94	$ 0.88	$ 0.85	$ 0.78	$ 0.82	$ 0.79	$ 1.62	$ 3.24	$ 3.18
Assuming Dilution	$ (1.11)	$ 0.87	$ 0.94	$ 0.88	$ 0.84	$ 0.78	$ 0.82	$ 0.79	$ 1.61	$ 3.23	$ 3.17