Equity Method Investments Schedule of Equity Method Investments (Details) - USD ($) $ in Millions |
1 Months Ended | 3 Months Ended | 9 Months Ended | ||
|---|---|---|---|---|---|
Feb. 11, 2015 |
Sep. 30, 2016 |
Sep. 30, 2015 |
Sep. 30, 2016 |
Sep. 30, 2015 |
|
| Schedule of Equity Method Investments [Line Items] | |||||
| Contributions to equity investees | $ 1,217.3 | ||||
| Millennium Pipeline | |||||
| Schedule of Equity Method Investments [Line Items] | |||||
| Contributions to equity investees | $ 4.3 | $ 1.4 | $ 6.2 | $ 1.4 | |
| Distributions of earnings from equity investees | 17.5 | 13.3 | 41.3 | 37.5 | |
| Hardy Storage | |||||
| Schedule of Equity Method Investments [Line Items] | |||||
| Contributions to equity investees | 0.0 | 0.0 | 0.0 | 0.0 | |
| Distributions of earnings from equity investees | 0.0 | 0.0 | 1.4 | 1.0 | |
| Pennant | |||||
| Schedule of Equity Method Investments [Line Items] | |||||
| Contributions to equity investees | 0.0 | 0.0 | 0.0 | 0.0 | |
| Distributions of earnings from equity investees | 2.3 | 2.9 | 8.3 | 5.6 | |
| Return of capital from equity investees | $ 0.8 | $ 0.2 | $ 1.6 | $ 2.4 | |