Equity Method Investments (Narrative) (Details) - USD ($) $ in Millions |
1 Months Ended | 3 Months Ended | 5 Months Ended | 6 Months Ended | ||
|---|---|---|---|---|---|---|
Feb. 11, 2015 |
Jun. 30, 2015 |
Jun. 30, 2014 |
Jun. 30, 2015 |
Jun. 30, 2015 |
Jun. 30, 2014 |
|
| Schedule of Equity Method Investments [Line Items] | ||||||
| Contributions | $ 1,217.3 | |||||
| Distributions | $ (9.2) | |||||
| Millennium Pipeline | ||||||
| Schedule of Equity Method Investments [Line Items] | ||||||
| Contributions | $ 0.0 | $ 0.0 | $ 0.0 | $ 2.6 | ||
| Distributions | 7.6 | 4.8 | 24.2 | 11.9 | ||
| Hardy Storage [Member] | ||||||
| Schedule of Equity Method Investments [Line Items] | ||||||
| Contributions | 0.0 | 0.0 | 0.0 | 0.0 | ||
| Distributions | 0.5 | 0.5 | 1.0 | 1.0 | ||
| Pennant [Member] | ||||||
| Schedule of Equity Method Investments [Line Items] | ||||||
| Contributions | 0.0 | 23.8 | 0.0 | 52.2 | ||
| Distributions | 1.5 | $ 0.0 | 2.7 | $ 0.0 | ||
| Proceeds from Equity Method Investment, Dividends or Distributions, Return of Capital | $ 0.9 | $ 2.2 | ||||