Fair Value: Asset and liabilities measured at fair value on a recurring basis (Details) - USD ($)	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014
Fair value measurement
Long-term investments	$ 86,771,000	$ 85,728,000
Interest rate options
Fair value measurement
Derivative assets	1,010,000	4,371,000
International equity trust
Fair value measurement
Unfunded commitments	$ 0
Notice period for redemptions	3 days
Recurring basis | Quoted Prices in Active Markets for Identical Assets (Level 1) | Domestic equity
Fair value measurement
Nuclear decommissioning trust funds	$ 151,178,000	159,536,000
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	74,698,000	68,854,000
Long-term investments	13,772,000	16,619,000
Recurring basis | Quoted Prices in Active Markets for Identical Assets (Level 1) | Other
Fair value measurement
Nuclear decommissioning trust funds	4,772,000	13,803,000
Long-term investments	479,000	118,000
Recurring basis | Quoted Prices in Active Markets for Identical Assets (Level 1) | Mutual funds
Fair value measurement
Long-term investments	48,649,000	51,741,000
Recurring basis | Significant Other Observable Inputs (Level 2) | Natural gas swaps
Fair value measurement
Derivative assets	24,995,000	18,914,000
Recurring basis | Significant Other Observable Inputs (Level 2) | International equity trust
Fair value measurement
Nuclear decommissioning trust funds	68,753,000	72,474,000
Long-term investments	12,846,000	11,162,000
Recurring basis | Significant Other Observable Inputs (Level 2) | Corporate bonds
Fair value measurement
Nuclear decommissioning trust funds	48,450,000	34,446,000
Long-term investments	9,903,000	5,445,000
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	15,503,000	16,148,000
Long-term investments	1,121,000	643,000
Recurring basis | Significant Other Observable Inputs (Level 2) | Municipal Bonds
Fair value measurement
Nuclear decommissioning trust funds		743,000
Recurring basis | Significant Unobservable Inputs (Level 3) | Interest rate options
Fair value measurement
Derivative assets	1,010,000	4,371,000
Recurring basis | Total Fair Value | Interest rate options
Fair value measurement
Derivative assets	1,010,000	4,371,000
Recurring basis | Total Fair Value | Natural gas swaps
Fair value measurement
Derivative assets	24,995,000	18,914,000
Recurring basis | Total Fair Value | Domestic equity
Fair value measurement
Nuclear decommissioning trust funds	151,178,000	159,536,000
Recurring basis | Total Fair Value | International equity trust
Fair value measurement
Nuclear decommissioning trust funds	68,753,000	72,474,000
Long-term investments	12,846,000	11,162,000
Recurring basis | Total Fair Value | Corporate bonds
Fair value measurement
Nuclear decommissioning trust funds	48,450,000	34,446,000
Long-term investments	9,903,000	5,445,000
Recurring basis | Total Fair Value | US Treasury and government agency securities
Fair value measurement
Nuclear decommissioning trust funds	75,173,000	68,854,000
Long-term investments	13,772,000	16,619,000
Recurring basis | Total Fair Value | Agency mortgage and asset backed securities
Fair value measurement
Nuclear decommissioning trust funds	15,503,000	16,148,000
Long-term investments	1,121,000	643,000
Recurring basis | Total Fair Value | Municipal Bonds
Fair value measurement
Nuclear decommissioning trust funds		743,000
Recurring basis | Total Fair Value | Other
Fair value measurement
Nuclear decommissioning trust funds	4,772,000	13,803,000
Long-term investments	479,000	118,000
Recurring basis | Total Fair Value | Mutual funds
Fair value measurement
Long-term investments	$ 48,649,000	$ 51,741,000