Condensed Balance Sheets Parenthetical - USD ($) shares in Millions, $ in Millions |
Jun. 30, 2018 |
Dec. 31, 2017 |
Jun. 30, 2017 |
|---|---|---|---|
| ASSETS | |||
| Accumulated depreciation | $ 2,537.0 | $ 2,333.1 | $ 2,117.6 |
| 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 | 167.0 | 167.1 | 166.3 |
| Common stock, outstanding | 167.0 | 167.1 | 166.3 |