Accumulated Other Comprehensive Loss (Narrative) (Details) - Millennium Pipeline - USD ($) $ in Millions |
6 Months Ended | |
|---|---|---|
Jun. 30, 2015 |
Jun. 30, 2014 |
|
| Accumulated Other Comprehensive Loss [Line Items] | ||
| Other Comprehensive Income Unrecognized Gain Loss On Derivatives Arising During Period Net Of Tax | $ 25.9 | $ 16.6 |
| Debt Instrument, Maturity Date | Jun. 01, 2025 | |