Taxes Other Than Income Taxes (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Taxes Other Than Income [Line Items]
Total taxes other than income	$ 14,712	$ 14,663	$ 43,094	$ 43,079
Property [Member]
Taxes Other Than Income [Line Items]
Total taxes other than income	7,007	6,945	20,119	20,776
Capital Stock [Member]
Taxes Other Than Income [Line Items]
Total taxes other than income	430	538	1,287	1,619
Gross Receipts, Excise And Franchise [Member]
Taxes Other Than Income [Line Items]
Total taxes other than income	2,979	2,859	8,181	7,855
Payroll [Member]
Taxes Other Than Income [Line Items]
Total taxes other than income	2,140	2,072	7,775	7,425
Regulatory Assessments [Member]
Taxes Other Than Income [Line Items]
Total taxes other than income	639	693	1,991	2,033
Other [Member]
Taxes Other Than Income [Line Items]
Total taxes other than income	$ 1,517	$ 1,556	$ 3,741	$ 3,371