Common Share Data (Details) - USD ($) $ / shares in Units, $ in Millions		3 Months Ended		9 Months Ended
		Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Earnings Per Share [Abstract]
Net income		$ 110.9	$ 125.9	$ 335.0	$ 331.4
Numerator for basic earnings per share - income (loss) available to common stockholders		110.9	125.9	335.0	331.4
Adjustment for interest expense incurred on assumed conversions of convertible senior notes		3.6	3.6	10.9	10.9
Numerator for diluted earnings per share - income (loss) available to common stockholders after assumed conversions		$ 114.6	$ 129.6	$ 345.9	$ 342.4
Denominator for basic earnings per share - weighted-average shares (a)	[1]	259,414,230	259,266,696	259,227,605	259,411,347
Effect of dilutive securities - stock based compensation awards		1,328,657	989,911	1,287,205	930,917
Effect of dilutive securities - convertible senior notes		35,701,545	35,611,510	35,678,280	35,588,263
Denominator for diluted earnings per shares adjusted weighted average shares and assumed conversions (a) (in shares)	[1]	296,444,432	295,868,117	296,193,090	295,930,527
Net income (loss) per share: Basic (in dollars per share)		$ 0.43	$ 0.48	$ 1.29	$ 1.28
Net income (loss) per share: Diluted (in dollars per share)		$ 0.39	$ 0.44	$ 1.17	$ 1.16
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive common stock equivalents excluded from earnings per share computations		1,678,675	5,040,050	1,678,675	5,040,050
Stock based compensation awards [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive common stock equivalents excluded from earnings per share computations		1,678,675	5,040,050	1,678,675	5,040,050
Convertible senior notes [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive common stock equivalents excluded from earnings per share computations		0	0	0	0

[1] (a) In calculating earnings per share, pertinent accounting rules require that common shares owned by the Company's Employee Savings and Stock Ownership Plan that are not yet allocated to participants in the plan be excluded from the calculation. Such shares are issued and outstanding and have the same voting and other rights applicable to all other common shares.