Quarterly Financial Data (Unaudited) (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Quarterly Financial Information Disclosure [Abstract]
Operating Revenues	$ 281,097	$ 122,427	$ 133,271	$ 350,201	$ 224,390	$ 128,808	$ 122,592	$ 255,631	$ 886,996	$ 731,421	$ 706,280
Expenses:
Cost of Sales - (Excluding depreciation)	153,794	76,073	78,947	228,138	139,703	89,160	87,827	152,301
Operations and Maintenance Including Fixed Charges	69,384	56,039	56,174	64,684	54,674	47,789	47,114	52,226
Income Taxes	10,415	(8,325)	(9,510)	11,869	(9,621)	(5,533)	(11,632)	7,772	4,449	(19,014)	11,479
Energy and Other Taxes	1,330	1,196	1,241	1,953	2,861	1,458	1,664	3,833	5,720	9,816	9,986
Total Expenses	234,923	124,983	126,852	306,644	187,617	132,874	124,973	216,132
Other Income and Expense	(2,182)	(1,720)	3,282	4,654	2,584	2,845	3,297	3,838
Income from Continuing Operations	43,992	(4,276)	9,701	48,211	39,357	(1,221)	916	43,337	97,628	82,389	92,776
Loss from Discontinued Operations - (Net of tax benefit)	(69)	(120)	(80)	(313)	(97)	(200)	(28)	(471)	(582)	(796)	(1,168)
Net Income (Loss)	$ 43,923	$ (4,396)	$ 9,621	$ 47,898	$ 39,260	$ (1,421)	$ 888	$ 42,866
Basic Earnings Per Common Share:
Continuing Operations (in dollars per share)	$ 1.31	$ (0.13)	$ 0.29	$ 1.47	$ 1.22	$ (0.04)	$ 0.03	$ 1.37	$ 2.95	$ 2.58	$ 3.02
Discontinued Operations (in dollars per share)	$ 0.00	$ 0.00	$ 0.00	$ (0.01)	$ 0.00	$ 0.00	$ 0.00	$ (0.02)	$ (0.02)	$ (0.03)	$ (0.04)
Basic Earnings Per Common Share (in dollars per share)	$ 1.31	$ (0.13)	$ 0.29	$ 1.46	$ 1.22	$ (0.04)	$ 0.03	$ 1.35	$ 2.93	$ 2.55	$ 2.98
Average Shares of Common Stock Outstanding - Basic (in shares)	33,652	33,166	32,963	32,765	32,259	31,984	31,949	31,757	33,139	31,989	30,744
Diluted Earnings Per Common Share:
Continuing Operations (in dollars per share)	$ 1.30	$ (0.13)	$ 0.29	$ 1.47	$ 1.22	$ (0.04)	$ 0.03	$ 1.36	$ 2.94	$ 2.57	$ 3.01
Discontinued Operations (in dollars per share)	$ 0.00	$ 0.00	$ 0.00	$ (0.01)	$ 0.00	$ 0.00	$ 0.00	$ (0.01)	$ (0.02)	$ (0.02)	$ (0.04)
Diluted Earnings Per Common Share (in dollars per share)	$ 1.30	$ (0.13)	$ 0.29	$ 1.46	$ 1.22	$ (0.04)	$ 0.03	$ 1.35	$ 2.92	$ 2.55	$ 2.97
Average Shares of Common Stock Outstanding - Diluted (in shares)	33,727	33,166	33,037	32,842	32,306	31,984	32,012	31,811	33,214	32,046	30,824