Quarterly financial data (unaudited) (Tables)

12 Months Ended

Dec. 31, 2020

Quarterly Financial Data [Abstract]

Summary of quarterly financial information

Summarized quarterly financial information for 2020 and 2019 is as follows:

	First Quarter		Second Quarter		Third Quarter		Fourth Quarter
	(dollars in thousands)
2020
Operating revenues	$	341,674	$	330,944	$	366,239	$	338,761
Operating margin	62,515		65,532		47,862		41,800
Net margin	23,204		26,162		8,471		(1,942)
2019
Operating revenues	$	356,600	$	358,860	$	382,623	$	332,209
Operating margin	66,032		51,712		59,209		40,256
Net margin	23,596		9,383		18,286		3,196