Condensed Balance Sheets Parenthetical - USD ($) shares in Millions, $ in Millions	Mar. 31, 2017	Dec. 31, 2016	Mar. 31, 2016
ASSETS
Accumulated depreciation	$ 2,001.1	$ 1,891.6	$ 1,587.9
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	165.9	165.4	165.2
Common stock, outstanding	165.9	165.4	165.2