Fair Value: - Asset and liabilities measured at fair value on a recurring basis (Details) - USD ($)	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015
Fair value measurement
Long-term investments	$ 99,874,000	$ 86,771,000
Interest rate options
Fair value measurement
Derivative assets	0	1,010,000
Natural gas swaps
Fair value measurement
Derivative liabilities	(22,848,000)	(22,848,000)
Derivative assets	15,090,000
International equity trust
Fair value measurement
Unfunded commitments	$ 0
Redemption notice period	3 days
Recurring basis | Total Fair Value | Interest rate options
Fair value measurement
Derivative assets		1,010,000
Recurring basis | Total Fair Value | Natural gas swaps
Fair value measurement
Derivative liabilities	$ (15,090,000)
Derivative assets		24,995,000
Recurring basis | Total Fair Value | Domestic equity
Fair value measurement
Nuclear decommissioning trust funds	170,408,000	151,178,000
Recurring basis | Total Fair Value | International equity trust
Fair value measurement
Nuclear decommissioning trust funds	66,861,000	68,753,000
Long-term investments	15,946,000	12,846,000
Recurring basis | Total Fair Value | Corporate bonds
Fair value measurement
Nuclear decommissioning trust funds	60,019,000	48,450,000
Long-term investments	11,853,000	9,903,000
Recurring basis | Total Fair Value | US Treasury and government agency securities
Fair value measurement
Nuclear decommissioning trust funds	65,725,000	75,173,000
Long-term investments	12,187,000	13,772,000
Recurring basis | Total Fair Value | Agency mortgage and asset backed securities
Fair value measurement
Nuclear decommissioning trust funds	17,410,000	15,503,000
Long-term investments	1,651,000	1,121,000
Recurring basis | Total Fair Value | Municipal Bonds
Fair value measurement
Nuclear decommissioning trust funds	943,000
Recurring basis | Total Fair Value | Mutual funds
Fair value measurement
Long-term investments	57,932,000	48,649,000
Recurring basis | Total Fair Value | Other
Fair value measurement
Nuclear decommissioning trust funds	4,663,000	4,772,000
Long-term investments	305,000	479,000
Recurring basis | Quoted Prices in Active Markets for Identical Assets (Level 1) | Domestic equity
Fair value measurement
Nuclear decommissioning trust funds	170,408,000	151,178,000
Recurring basis | Quoted Prices in Active Markets for Identical Assets (Level 1) | US Treasury and government agency securities
Fair value measurement
Nuclear decommissioning trust funds	65,725,000	74,698,000
Long-term investments	12,187,000	13,772,000
Recurring basis | Quoted Prices in Active Markets for Identical Assets (Level 1) | Mutual funds
Fair value measurement
Long-term investments	57,932,000	48,649,000
Recurring basis | Quoted Prices in Active Markets for Identical Assets (Level 1) | Other
Fair value measurement
Nuclear decommissioning trust funds	4,663,000	4,772,000
Long-term investments	305,000	479,000
Recurring basis | Significant Other Observable Inputs (Level 2) | Natural gas swaps
Fair value measurement
Derivative liabilities	(15,090,000)
Derivative assets		24,995,000
Recurring basis | Significant Other Observable Inputs (Level 2) | International equity trust
Fair value measurement
Nuclear decommissioning trust funds	66,861,000	68,753,000
Long-term investments	15,946,000	12,846,000
Recurring basis | Significant Other Observable Inputs (Level 2) | Corporate bonds
Fair value measurement
Nuclear decommissioning trust funds	60,019,000	48,450,000
Long-term investments	11,853,000	9,903,000
Recurring basis | Significant Other Observable Inputs (Level 2) | US Treasury and government agency securities
Fair value measurement
Nuclear decommissioning trust funds		475,000
Recurring basis | Significant Other Observable Inputs (Level 2) | Agency mortgage and asset backed securities
Fair value measurement
Nuclear decommissioning trust funds	17,410,000	15,503,000
Long-term investments	1,651,000	1,121,000
Recurring basis | Significant Other Observable Inputs (Level 2) | Municipal Bonds
Fair value measurement
Nuclear decommissioning trust funds	$ 943,000
Recurring basis | Significant Unobservable Inputs (Level 3) | Interest rate options
Fair value measurement
Derivative assets		$ 1,010,000