Fair Value - Asset and liabilities measured at fair value on a recurring basis (Details) - USD ($)	6 Months Ended
	Jun. 30, 2019	Dec. 31, 2018
Fair value
Nuclear decommissioning trust fund	$ 478,976,000	$ 420,818,000
Long-term investments	189,354,000	164,125,000
Natural gas swaps
Fair value
Derivative liabilities	23,000,000	13,154,000
International equity trust
Fair value
Unfunded commitments	$ 0
Redemption notice period	3 days
Recurring basis | Natural gas swaps
Fair value
Derivative liabilities	$ 23,000,000	13,154,000
Recurring basis | Domestic equity
Fair value
Nuclear decommissioning trust fund	161,570,000	136,196,000
Recurring basis | International equity trust
Fair value
Nuclear decommissioning trust fund	90,317,000	76,852,000
Long-term investments	21,734,000	17,382,000
Recurring basis | Corporate bonds and debt
Fair value
Nuclear decommissioning trust fund	60,809,000	51,356,000
Long-term investments	15,861,000	12,571,000
Recurring basis | US Treasury securities
Fair value
Nuclear decommissioning trust fund	51,422,000	47,712,000
Long-term investments	12,792,000	12,062,000
Recurring basis | Mortgage backed securities
Fair value
Nuclear decommissioning trust fund	54,478,000	56,004,000
Long-term investments	9,867,000	11,517,000
Recurring basis | Domestic mutual funds
Fair value
Nuclear decommissioning trust fund	51,820,000	43,359,000
Long-term investments	89,879,000	94,494,000
Recurring basis | Municipal bonds
Fair value
Nuclear decommissioning trust fund	316,000	278,000
Recurring basis | Federal agency securities
Fair value
Nuclear decommissioning trust fund	2,541,000	6,066,000
Long-term investments	767,000	941,000
Recurring basis | Non-US Gov't bonds & private placements
Fair value
Nuclear decommissioning trust fund	265,000	964,000
Recurring basis | Treasury STRIPS
Fair value
Long-term investments	36,043,000	14,113,000
Recurring basis | Other
Fair value
Nuclear decommissioning trust fund	5,438,000	2,031,000
Long-term investments	2,411,000	1,045,000
Recurring basis | Quoted Prices in Active Markets for Identical Assets (Level 1) | Domestic equity
Fair value
Nuclear decommissioning trust fund	161,570,000	136,196,000
Recurring basis | Quoted Prices in Active Markets for Identical Assets (Level 1) | US Treasury securities
Fair value
Nuclear decommissioning trust fund	51,422,000	47,712,000
Long-term investments	12,792,000	12,062,000
Recurring basis | Quoted Prices in Active Markets for Identical Assets (Level 1) | Domestic mutual funds
Fair value
Nuclear decommissioning trust fund	51,820,000	43,359,000
Long-term investments	89,879,000	94,494,000
Recurring basis | Quoted Prices in Active Markets for Identical Assets (Level 1) | Other
Fair value
Nuclear decommissioning trust fund	5,438,000	2,031,000
Long-term investments	2,411,000	1,045,000
Recurring basis | Significant Other Observable Inputs (Level 2) | Natural gas swaps
Fair value
Derivative liabilities	23,000,000	13,154,000
Recurring basis | Significant Other Observable Inputs (Level 2) | International equity trust
Fair value
Nuclear decommissioning trust fund	90,317,000	76,852,000
Long-term investments	21,734,000	17,382,000
Recurring basis | Significant Other Observable Inputs (Level 2) | Corporate bonds and debt
Fair value
Nuclear decommissioning trust fund	60,809,000	48,853,000
Long-term investments	15,861,000	11,366,000
Recurring basis | Significant Other Observable Inputs (Level 2) | Mortgage backed securities
Fair value
Nuclear decommissioning trust fund	54,478,000	56,004,000
Long-term investments	9,867,000	11,517,000
Recurring basis | Significant Other Observable Inputs (Level 2) | Municipal bonds
Fair value
Nuclear decommissioning trust fund	316,000	278,000
Recurring basis | Significant Other Observable Inputs (Level 2) | Federal agency securities
Fair value
Nuclear decommissioning trust fund	2,541,000	6,066,000
Long-term investments	767,000	941,000
Recurring basis | Significant Other Observable Inputs (Level 2) | Non-US Gov't bonds & private placements
Fair value
Nuclear decommissioning trust fund	265,000	964,000
Recurring basis | Significant Other Observable Inputs (Level 2) | Treasury STRIPS
Fair value
Long-term investments	$ 36,043,000	14,113,000
Recurring basis | Significant Unobservable Inputs (Level 3) | Corporate bonds and debt
Fair value
Nuclear decommissioning trust fund		2,503,000
Long-term investments		$ 1,205,000