Summary of Significant Accounting Policies (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Summary of Significant Accounting Policies [Abstract]
Pension and post-retirement benefits	$ 9,470	$ 11,455
Unrecovered Income Taxes - Regulatory Asset	75,763	66,631
Deferred Revenue	10,408	6,813
Other Regulatory Assets	4,931	5,202
Regulatory Assets	100,572	90,101
Other Regulated Liabilities, Current	1,334	1,117
Unamortized Investment Tax Credits	1,057	1,133
Customer Refund Liability, Total	2,865	64
Deferred Future Income Taxes and Other	67,725	58,384
Excess Accumulated Deferred Income Tax	29,611	30,937
Regulatory Liabilities	$ 102,592	$ 91,635