Income Tax (Tables)

3 Months Ended

Mar. 31, 2018

Income Tax Disclosure [Abstract]

Schedule of Effective Income Tax Rate Reconciliation

The effective income tax rate varied from the combined federal and state statutory tax rates due to the following:

	Three Months Ended March 31,
Dollars in thousands	2018			2017
Income taxes at statutory rates (federal and state)	$	15,198		$	26,600
Increase (decrease):
Differences required to be flowed-through by regulatory commissions	849			1,518
Other, net	(585		)	(1,195		)
Total provision for income taxes	$	15,462		$	26,923
Effective tax rate	27.1		%	40.0		%