Accounting for Derivative Instruments and Hedging Activities (Offsetting) (Details) - Energy derivative contracts - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Assets:
Gross Amounts Recognized in the Statement of Financial Position 1	$ 49,019	$ 24,405
Gross Amounts Offset in the Statement of Financial Position	0	0
Net of Amounts Presented in the Statement of Financial Position	49,019	24,405
Commodity Contracts	25,388	17,940
Cash Collateral Received/Posted	0	0
Net Amount	23,631	6,465
Liabilities:
Gross Amounts Recognized in the Statement of Financial Position 1	57,756	86,094
Gross Amounts Offset in the Statement of Financial Position	0	0
Net of Amounts Presented in the Statement of Financial Position	57,756	86,094
Commodity Contracts	25,388	17,940
Cash Collateral Received/Posted	0	353
Net Amount	$ 32,368	$ 67,801