Selected Quarterly Financial Information (unaudited) (Quarterly Information) (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Quarterly Financial Information Disclosure [Abstract]
Revenues	$ 914.0	$ 1,055.0	$ 941.3	$ 723.8	$ 885.0	$ 1,032.9	$ 1,099.1	$ 790.3	$ 3,634.1	$ 3,807.2	$ 3,883.9
Gross Profit	275.3	323.3	275.7	165.7	247.8	298.3	332.1	201.6	1,040.1	1,079.8	1,111.2
Net Income (Loss)	$ 111.7	$ 131.7	$ 100.0	$ 12.9	$ 114.0	$ 114.7	$ 110.7	$ 69.3	$ 356.3	$ 408.7	$ 359.0
Basic Earnings (Loss) per Share (in usd per share)	$ 2.92	$ 3.44	$ 2.62	$ 0.34	$ 2.96	$ 2.97	$ 2.83	$ 1.75	$ 9.30	$ 10.49	$ 8.84
Diluted Earnings (Loss) per Share (in usd per share)	2.89	3.42	2.60	0.33	2.93	2.94	2.80	1.73	9.24	10.38	8.74
Cash Dividends per Common Share (in usd per share)	$ 0.77	$ 0.77	$ 0.77	$ 0.77	$ 0.77	$ 0.77	$ 0.77	$ 0.64	$ 3.08	$ 2.95	$ 2.43