Net Income Per Common Share (Detail) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Numerator
Net Income (Loss)	$ 242.0	$ (284.7)	$ 804.1	$ 274.3
Denominator
Weighted Average Common Shares - Basic	208,097,800	218,892,800	211,131,500	220,513,100
Dilution for Assumed Exercises of Stock Options and Nonvested Stock Awards	18,200	0	65,700	433,200
Weighted Average Common Shares - Assuming Dilution	208,116,000.0	218,892,800	211,197,200	220,946,300
Net Income (Loss) Per Common Share
Basic	$ 1.16	$ (1.30)	$ 3.81	$ 1.24
Assuming Dilution	$ 1.16	$ (1.30)	$ 3.81	$ 1.24