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Employee Benefit Plans: (Details) - USD ($)
$ in Thousands
 3 Months Ended 9 Months Ended
Sep. 30, 2016
Sep. 30, 2015
Sep. 30, 2016
Sep. 30, 2015
Pension Benefits        
Components of Net Periodic Benefits Cost:        
Service cost $ 1,274 $ 1,569 $ 3,822 $ 4,707
Interest cost 1,978 1,922 5,934 5,766
Expected return on plan assets (2,457) (2,449) (7,377) (7,347)
Amortization of prior service cost (benefit) 12 30 36 90
Amortization of actuarial (gain) loss 228 448 684 1,344
Net periodic pension cost under accounting standards 1,035 1,520 3,099 4,560
Regulatory adjustment — deferred 221 130 644 392
Total expense recognized, before allocation to overhead pool 1,256 1,650 3,743 4,952
Expected contributions in current fiscal year 5,300   5,300  
Pension Benefits | GSWC        
Components of Net Periodic Benefits Cost:        
Regulatory adjustment — deferred     1,500  
Other Postretirement Benefits        
Components of Net Periodic Benefits Cost:        
Service cost 68 65 204 255
Interest cost 97 99 291 327
Expected return on plan assets (122) (123) (366) (369)
Amortization of prior service cost (benefit) (9) (50) (27) (150)
Amortization of actuarial (gain) loss (150) (131) (450) (237)
Net periodic pension cost under accounting standards (116) (140) (348) (174)
Regulatory adjustment — deferred 0 0 0 0
Total expense recognized, before allocation to overhead pool (116) (140) (348) (174)
SERP        
Components of Net Periodic Benefits Cost:        
Service cost 200 204 600 612
Interest cost 186 163 558 489
Expected return on plan assets 0 0 0 0
Amortization of prior service cost (benefit) 6 29 18 87
Amortization of actuarial (gain) loss 73 108 219 324
Net periodic pension cost under accounting standards 465 504 1,395 1,512
Regulatory adjustment — deferred 0 0 0 0
Total expense recognized, before allocation to overhead pool $ 465 $ 504 $ 1,395 $ 1,512