Derivative Instruments and Hedging Activities - Recognized Gain (Loss ) on Derivative Instruments (Details) - USD ($) $ in Millions |
12 Months Ended | ||
|---|---|---|---|
Dec. 31, 2016 |
Dec. 31, 2015 |
Dec. 31, 2014 |
|
| Commodity Derivative Instruments | |||
| Recognized Net Gain (Loss) on Derivative Instruments | |||
| Net (losses) gains recognized on derivative instruments | $ 51.3 | $ 158.8 | $ 189.6 |