Employee Benefit Plans: (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Pension Benefits
Components of Net Periodic Benefits Cost:
Service cost	$ 1,569	$ 1,411	$ 4,707	$ 4,233
Interest cost	1,922	1,880	5,766	5,640
Expected return on plan assets	(2,449)	(2,225)	(7,347)	(6,675)
Amortization of transition	0	0	0	0
Amortization of prior service cost (benefit)	30	30	90	89
Amortization of actuarial (gain) loss	448	0	1,344	0
Net periodic pension cost under accounting standards	1,520	1,096	4,560	3,287
Regulatory adjustment — deferred	130	374	392	1,123
Total expense recognized, before allocation to overhead pool	1,650	1,470	4,952	4,410
Defined Benefit Plan, Contributions by Employer	6,000		7,000
Pension Benefits | GSWC
Components of Net Periodic Benefits Cost:
Regulatory adjustment — deferred			2,300
Other Postretirement Benefits
Components of Net Periodic Benefits Cost:
Service cost	65	63	255	261
Interest cost	99	111	327	371
Expected return on plan assets	(123)	(114)	(369)	(340)
Amortization of transition	0	105	0	313
Amortization of prior service cost (benefit)	(50)	(50)	(150)	(150)
Amortization of actuarial (gain) loss	(131)	(115)	(237)	(247)
Net periodic pension cost under accounting standards	(140)	0	(174)	208
Regulatory adjustment — deferred	0	0	0	0
Total expense recognized, before allocation to overhead pool	(140)	0	(174)	208
SERP
Components of Net Periodic Benefits Cost:
Service cost	204	192	612	576
Interest cost	163	154	489	462
Expected return on plan assets	0	0	0	0
Amortization of transition	0	0	0	0
Amortization of prior service cost (benefit)	29	40	87	120
Amortization of actuarial (gain) loss	108	35	324	105
Net periodic pension cost under accounting standards	504	421	1,512	1,263
Regulatory adjustment — deferred	0	0	0	0
Total expense recognized, before allocation to overhead pool	$ 504	$ 421	$ 1,512	$ 1,263