Financial Instruments and Investments (Carrying Amount and Fair Value Amounts) (Details) - USD ($) $ in Thousands		Dec. 31, 2017	Dec. 31, 2016
Carrying (Reported) Amount, Fair Value Disclosure
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Debt Instrument, Fair Value Disclosure		$ 1,369,521	$ 1,360,230
Carrying (Reported) Amount, Fair Value Disclosure | Pollution Control Bonds
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Debt Instrument, Fair Value Disclosure	[1]	157,676	190,775
Carrying (Reported) Amount, Fair Value Disclosure | Senior Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Debt Instrument, Fair Value Disclosure		993,426	993,086
Carrying (Reported) Amount, Fair Value Disclosure | RGRT Senior Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Debt Instrument, Fair Value Disclosure	[2]	44,886	94,795
Carrying (Reported) Amount, Fair Value Disclosure | Revolving Credit Facility
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Debt Instrument, Fair Value Disclosure	[2]	173,533	81,574
Estimate of Fair Value Measurement
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Debt Instrument, Fair Value Disclosure		1,601,711	1,499,532
Estimate of Fair Value Measurement | Pollution Control Bonds
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Debt Instrument, Fair Value Disclosure	[1]	169,186	206,818
Estimate of Fair Value Measurement | Senior Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Debt Instrument, Fair Value Disclosure		1,211,922	1,112,285
Estimate of Fair Value Measurement | RGRT Senior Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Debt Instrument, Fair Value Disclosure	[2]	47,070	98,855
Estimate of Fair Value Measurement | Revolving Credit Facility
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Debt Instrument, Fair Value Disclosure	[2]	$ 173,533	$ 81,574

[1] In September 2017, the $33.3 million 2012 Series A 1.875% Pollution Control Bonds which were subject to mandatory tender for purchase were redeemed and retired utilizing funds borrowed under the RCF.

[2] Nuclear fuel financing, as of December 31, 2017 and December 31, 2016, is funded through $45 million and $95 million RGRT Senior Notes and $88.5 million and $37.6 million, respectively under the RCF. In August 2017, RGRT's $50.0 million Series B 4.47% Senior Notes matured and were paid utilizing funds borrowed under the RCF. As of December 31, 2017, $85.0 million was outstanding under the RCF for working capital or general corporate purposes. As of December 31, 2016, $44.0 million amount was outstanding under the RCF for working capital or general corporate purposes. The interest rate on the Company’s borrowings under the RCF is reset throughout the period reflecting current market rates. Consequently, the carrying value approximates fair value