Unaudited Quarterly Financial Information (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Operating revenue	$ 67,262	$ 85,238	$ 74,230	$ 50,139	$ 62,449	$ 82,374	$ 65,575	$ 51,149	$ 276,869	$ 261,547	$ 238,955
Operating Income (Loss)	11,351	19,346	16,937	5,773	14,110	20,877	13,585	6,725	53,407	55,297	54,391
Net income	4,677	8,950	7,440	1,317	5,924	10,084	5,201	1,109	22,384	22,318	20,878
Comprehensive income	5,304	9,135	7,351	1,671	5,871	10,125	5,260	1,098	23,461	22,354	20,793
Earnings per share
—Basic (in dollars per share)	$ 0.23	$ 0.44	$ 0.37	$ 0.07	$ 0.32	$ 0.54	$ 0.28	$ 0.06	$ 1.13	$ 1.20	$ 1.12
—Diluted (in dollars per share)	$ 0.23	$ 0.44	$ 0.37	$ 0.07	$ 0.31	$ 0.53	$ 0.28	$ 0.06	$ 1.12	$ 1.18	$ 1.11
Dividends per share	$ 0.18	$ 0.18	$ 0.18	$ 0.18	$ 0.18	$ 0.18	$ 0.18	$ 0.18
Balancing and memorandum accounts, revenue and regulatory assets									$ 5,019	$ 2,604
--- High (in dollars per share) [Member]
Earnings per share
Market Price, Stock	$ 30.03	$ 28.52	$ 27.66	$ 28.09	$ 26.62	$ 25.64	$ 24.34	$ 24.91
--- Low (in dollars per share) [Member]
Earnings per share
Market Price, Stock	$ 26.40	$ 25.62	$ 24.58	$ 25.70	$ 22.69	$ 22.95	$ 22.81	$ 22.96