Income Taxes Income Taxes (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Income Tax Disclosure [Abstract]
Federal income taxes at statutory rates	$ 30,639	$ 13,790
Federal income taxes at statutory rates	21.00%	21.00%
Tax effect of regulatory treatment of utility plant differences	$ (2,080)	$ (1,018)
Tax effect of regulatory treatment of utility plant differences	(1.40%)	(1.60%)
State income tax expense	$ 1,659	$ 964
State income tax expense	1.10%	1.50%
Acquisition costs	$ (1,824)	$ 46
Acquisition costs	(1.20%)	0.10%
Settlement of equity awards	$ 612	$ (990)
Settlement of equity awards	0.40%	(1.50%)
Other	$ 1,011	$ (2,082)
Other	0.70%	(3.20%)
Income Tax Expense (Benefit)	$ 30,017	$ 10,710
Effective Income Tax Rate	20.60%	16.30%