Income Taxes (Income Tax Provisions) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Income Tax Examination [Line Items]
Federal income tax expense computed on income at statutory rate	$ 23,243	$ 52,243	$ 52,740
Difference due to:
State taxes, net of federal benefit	3,059	597	2,633
AEFUDC	(182)	450	(475)
Permanent tax differences	(682)	(2,562)	(2,369)
Other	930	276	1,389
Total income tax expense	$ 26,368	$ 51,004	$ 53,918
Effective income tax rate	23.80%	34.20%	35.80%