Gain on Sale of Assets (Details) - USD ($) $ in Millions |
12 Months Ended | ||
|---|---|---|---|
Dec. 31, 2015 |
Dec. 31, 2014 |
Dec. 31, 2013 |
|
| Property, Plant and Equipment [Line Items] | |||
| Gain on Conveyances | $ 52.3 | $ 34.5 | $ 7.3 |
| Gain on Conveyances Cash Received | 35.8 | ||
| Deferred Gains On Conveyances | 8.1 | $ 19.6 | |
| Gain (Loss) on Disposition of Property Plant Equipment | $ 54.7 | ||
| Gas Stored Base Gas | |||
| Property, Plant and Equipment [Line Items] | |||
| Gain (Loss) on Disposition of Property Plant Equipment | $ 11.1 | ||