Fair Value of Financial Assets and Liabilities, Derivative Instruments (Details) - MMBTU MMBTU in Thousands		Jun. 30, 2018	Dec. 31, 2017
Natural Gas Commodity (in million British thermal units)
Gross Notional Amounts of Commodity Options [Abstract]
Derivative, Nonmonetary Notional amount	[1],[2]	58	42

[1] Amounts are not reflective of net positions in the underlying commodities.

[2] Notional amounts for options are included on a gross basis, but are weighted for the probability of exercise.