Fair Value - Asset and liabilities measured at fair value on a recurring basis (Details) - USD ($)	9 Months Ended
	Sep. 30, 2018	Dec. 31, 2017
Fair value
Nuclear decommissioning trust fund	$ 460,974,000	$ 445,055,000
Long-term investments	165,806,000	140,622,000
Natural gas swaps
Fair value
Derivative liabilities	14,961,000	6,328,000
International equity trust
Fair value
Unfunded commitments	$ 0
Redemption notice period	3 days
Recurring basis | Natural gas swaps
Fair value
Derivative liabilities	$ 14,961,000	6,328,000
Recurring basis | Domestic equity
Fair value
Nuclear decommissioning trust fund	157,382,000	142,419,000
Recurring basis | International equity trust
Fair value
Nuclear decommissioning trust fund	87,462,000	88,820,000
Long-term investments	19,776,000	20,071,000
Recurring basis | Corporate bonds and debt
Fair value
Nuclear decommissioning trust fund	59,134,000	66,317,000
Long-term investments	15,018,000	16,215,000
Recurring basis | US Treasury securities
Fair value
Nuclear decommissioning trust fund	35,578,000	38,791,000
Long-term investments	8,084,000	6,670,000
Recurring basis | Mortgage backed securities
Fair value
Nuclear decommissioning trust fund	58,788,000	49,379,000
Long-term investments	12,410,000	7,267,000
Recurring basis | Domestic mutual funds
Fair value
Nuclear decommissioning trust fund	52,994,000	47,833,000
Long-term investments	100,175,000	87,011,000
Recurring basis | Municipal bonds
Fair value
Nuclear decommissioning trust fund	283,000	92,000
Recurring basis | Federal agency securities
Fair value
Nuclear decommissioning trust fund	6,832,000	3,725,000
Long-term investments	1,002,000	259,000
Recurring basis | Non-US Gov't bonds & private placements
Fair value
Nuclear decommissioning trust fund	1,119,000
Recurring basis | Treasury STRIPS
Fair value
Long-term investments	9,223,000
Recurring basis | Other
Fair value
Nuclear decommissioning trust fund	1,402,000	7,679,000
Long-term investments	118,000	3,129,000
Recurring basis | Quoted Prices in Active Markets for Identical Assets (Level 1) | Domestic equity
Fair value
Nuclear decommissioning trust fund	157,382,000	142,419,000
Recurring basis | Quoted Prices in Active Markets for Identical Assets (Level 1) | US Treasury securities
Fair value
Nuclear decommissioning trust fund	35,578,000	38,791,000
Long-term investments	8,084,000	6,670,000
Recurring basis | Quoted Prices in Active Markets for Identical Assets (Level 1) | Domestic mutual funds
Fair value
Nuclear decommissioning trust fund	52,994,000	47,833,000
Long-term investments	100,175,000	87,011,000
Recurring basis | Quoted Prices in Active Markets for Identical Assets (Level 1) | Other
Fair value
Nuclear decommissioning trust fund	1,402,000	7,679,000
Long-term investments	118,000	3,129,000
Recurring basis | Significant Other Observable Inputs (Level 2) | Natural gas swaps
Fair value
Derivative liabilities	14,961,000	6,328,000
Recurring basis | Significant Other Observable Inputs (Level 2) | International equity trust
Fair value
Nuclear decommissioning trust fund	87,462,000	88,820,000
Long-term investments	19,776,000	20,071,000
Recurring basis | Significant Other Observable Inputs (Level 2) | Corporate bonds and debt
Fair value
Nuclear decommissioning trust fund	56,276,000	66,317,000
Long-term investments	13,535,000	16,215,000
Recurring basis | Significant Other Observable Inputs (Level 2) | Mortgage backed securities
Fair value
Nuclear decommissioning trust fund	58,788,000	49,379,000
Long-term investments	12,410,000	7,267,000
Recurring basis | Significant Other Observable Inputs (Level 2) | Municipal bonds
Fair value
Nuclear decommissioning trust fund	283,000	92,000
Recurring basis | Significant Other Observable Inputs (Level 2) | Federal agency securities
Fair value
Nuclear decommissioning trust fund	6,832,000	3,725,000
Long-term investments	1,002,000	$ 259,000
Recurring basis | Significant Other Observable Inputs (Level 2) | Non-US Gov't bonds & private placements
Fair value
Nuclear decommissioning trust fund	1,119,000
Recurring basis | Significant Other Observable Inputs (Level 2) | Treasury STRIPS
Fair value
Long-term investments	9,223,000
Recurring basis | Significant Unobservable Inputs (Level 3) | Corporate bonds and debt
Fair value
Nuclear decommissioning trust fund	2,858,000
Long-term investments	$ 1,483,000