Quarterly financial data (unaudited) (Tables)

12 Months Ended

Dec. 31, 2022

Quarterly Financial Data [Abstract]

Summary of quarterly financial information

	First		Second		Third		Fourth
	Quarter		Quarter		Quarter		Quarter
	(dollars in thousands)
2022
Operating revenues	$	420,442	$	533,128	$	704,265	$	472,302
Operating margin		57,845		54,269		64,553		17,384
Net margin		21,980		18,167		32,097		(10,540)
2021
Operating revenues	$	376,331	$	358,127	$	460,822	$	409,583
Operating margin		64,573		53,035		46,025		30,748
Net margin		25,958		13,145		8,907		9,771