Derivative Instruments (Net Cash Received (Paid) Related to Derivatives) (Schedule) (Details) - USD ($) $ in Millions |
6 Months Ended | |
|---|---|---|
Jun. 30, 2018 |
Jun. 30, 2017 |
|
| Derivative Instruments and Hedging Activities Disclosure [Abstract] | ||
| Operating activities | $ 246.1 | $ 89.5 |
| Investing activities | 0.0 | (0.5) |
| Financing activities | 10.2 | (139.0) |
| Total | $ 256.3 | $ (50.0) |