Accumulated Other Comprehensive Loss (Narrative) (Details) - USD ($) $ in Millions |
9 Months Ended | |
|---|---|---|
Sep. 30, 2016 |
Dec. 31, 2015 |
|
| Accumulated Other Comprehensive Loss [Line Items] | ||
| Other Comprehensive Income Unrecognized Gain Loss On Derivatives Arising During Period Before Tax | $ 23.7 | $ 25.0 |
| Millennium Pipeline | ||
| Accumulated Other Comprehensive Loss [Line Items] | ||
| Unrealized loss, amortization period | 15 years | |
| Debt Instrument, Maturity Date | Jun. 01, 2025 |