Quarterly Financial Data (Unaudited) (Details) $ / shares in Units, shares in Thousands, $ in Thousands			3 Months Ended								12 Months Ended
	May 08, 2015	Feb. 26, 2015	Dec. 31, 2016 USD ($) $ / shares shares	Sep. 30, 2016 USD ($) $ / shares shares	Jun. 30, 2016 USD ($) $ / shares shares	Mar. 31, 2016 USD ($) $ / shares shares	Dec. 31, 2015 USD ($) $ / shares shares	Sep. 30, 2015 USD ($) $ / shares shares	Jun. 30, 2015 USD ($) $ / shares shares	Mar. 31, 2015 USD ($) $ / shares shares	Dec. 31, 2016 USD ($) $ / shares shares	Dec. 31, 2015 USD ($) $ / shares shares	Dec. 31, 2014 USD ($) $ / shares shares
Quarterly Financial Information Disclosure [Abstract]
Operating Revenues			$ 329,981	$ 219,082	$ 154,402	$ 333,035	$ 257,844	$ 141,062	$ 177,710	$ 382,952	$ 1,036,500	$ 959,568	$ 886,996
Expenses:
Cost of Sales - (Excluding depreciation)			186,684	142,988	98,340	152,975	127,566	99,124	90,945	241,707
Operations and Maintenance Including Fixed Charges			77,858	69,410	71,034	73,042	71,148	63,867	62,634	71,279
Income Taxes			19,266	2,807	(7,189)	39,267	(1,006)	(10,968)	(3,279)	16,613	54,151	1,360	4,449
Energy and Other Taxes			1,725	1,449	1,243	1,925	1,334	1,231	1,265	2,196	6,342	6,026	5,720
Total Expenses			285,533	216,654	163,428	267,209	199,042	153,254	151,565	331,795
Other Income and Expense			1,560	7,236	4,228	2,361	(7,854)	(340)	(12,804)	2,696
Income from Continuing Operations			46,008	9,664	(4,798)	68,187	50,948	(12,532)	13,341	53,853	119,061	105,610	97,628
Loss from Discontinued Operations - (Net of tax benefit)			(75)	(29)	(29)	(118)	(62)	(91)	(74)	(276)	$ (251)	$ (503)	$ (582)
Net Income (Loss)			$ 45,933	$ 9,635	$ (4,827)	$ 68,069	$ 50,886	$ (12,623)	$ 13,267	$ 53,577
Basic Earnings Per Common Share:
Continuing Operations (in USD per share) | $ / shares			$ 0.58	$ 0.12	$ (0.06)	$ 0.96	$ 0.73	$ (0.18)	$ 0.19	$ 0.79	$ 1.56	$ 1.54	$ 1.47
Discontinued Operations (in USD per share) | $ / shares			0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.01	0.01
Basic Earnings per Common Share (in USD per share) | $ / shares			$ 0.58	$ 0.12	$ (0.06)	$ 0.96	$ 0.73	$ (0.18)	$ 0.19	$ 0.79	$ 1.56	$ 1.53	$ 1.46
Average Shares of Common Stock Outstanding - Basic (in shares) | shares			79,478	79,478	75,298	71,127	69,460	68,607	68,467	68,397	76,362	68,735	66,278
Diluted Earnings Per Common Share:
Continuing Operations (in USD per share) | $ / shares			$ 0.58	$ 0.12	$ (0.06)	$ 0.95	$ 0.73	$ (0.18)	$ 0.19	$ 0.79	$ 1.56	$ 1.53	$ 1.47
Discontinued Operations (in USD per share) | $ / shares			0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.01	0.01
Diluted Earnings per Common Share (in USD per share) | $ / shares			$ 0.58	$ 0.12	$ (0.06)	$ 0.95	$ 0.73	$ (0.18)	$ 0.19	$ 0.79	$ 1.56	$ 1.52	$ 1.46
Average Shares of Common Stock Outstanding - Diluted (in shares) | shares			79,643	79,635	75,298	71,416	69,646	68,607	68,653	68,618	76,475	68,931	66,428
Stock split, conversion ratio	2	2