Accounting for derivative instruments - forecasted trans (Details) MMBTU in Millions, $ in Millions, J in Billions |
12 Months Ended | |
|---|---|---|
|
Dec. 31, 2015
USD ($)
MMBTU
J
|
Dec. 31, 2014
USD ($)
MMBTU
J
|
|
| Natural gas swaps | ||
| Derivative instruments | ||
| Volume of forecasted transactions, energy (swaps: Mmbtu, agreements: joules) | MMBTU | 2.8 | 6.3 |
| Gas purchase agreements | ||
| Derivative instruments | ||
| Volume of forecasted transactions, energy (swaps: Mmbtu, agreements: joules) | J | 25.0 | 33.9 |
| Interest rate swaps | ||
| Derivative instruments | ||
| Volume of forecasted transactions, (in dollars) | $ | $ 302.3 | $ 333.9 |