Net Income (Loss) Per Common Share (Detail) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Numerator
Net Income (Loss)	$ (284.7)	$ 252.3	$ 274.3	$ 727.3
Denominator (000s)
Weighted Average Common Shares - Basic	218,892,800	225,288,100	220,513,100	227,375,500
Dilution for Assumed Exercises of Stock Options and Nonvested Stock Awards	0	741,700	433,200	805,000
Weighted Average Common Shares - Assuming Dilution	218,892,800	226,029,800	220,946,300	228,180,500
Net Income (Loss) Per Common Share
Basic	$ (1.30)	$ 1.12	$ 1.24	$ 3.20
Assuming Dilution	$ (1.30)	$ 1.12	$ 1.24	$ 3.19