Equity Method Investments Schedule of Equity Method Investments (Details) - USD ($) $ in Millions |
1 Months Ended | 3 Months Ended | |
|---|---|---|---|
Feb. 11, 2015 |
Mar. 31, 2016 |
Mar. 31, 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 | $ 1.9 | $ 0.0 | |
| Distributions of earnings from equity investees | 15.2 | 16.6 | |
| Hardy Storage | |||
| Schedule of Equity Method Investments [Line Items] | |||
| Contributions to equity investees | 0.0 | 0.0 | |
| Distributions of earnings from equity investees | 0.7 | 0.5 | |
| Pennant | |||
| Schedule of Equity Method Investments [Line Items] | |||
| Contributions to equity investees | 0.0 | 0.0 | |
| Distributions of earnings from equity investees | 3.0 | 1.2 | |
| Return of capital from equity investees | $ 0.2 | $ 1.3 | |