Condensed Balance Sheets Parenthetical - USD ($) shares in Millions, $ in Millions	Mar. 31, 2018	Dec. 31, 2017	Mar. 31, 2017
ASSETS
Accumulated depreciation	$ 2,444.2	$ 2,333.1	$ 2,001.1
Shareholders' equity:
Common stock, par value	$ 1.00	$ 1.00	$ 1.00
Common stock, authorized (in shares)	240.0	240.0	240.0
Common stock, issued	167.2	167.1	165.9
Common stock, outstanding	167.2	167.1	165.9