Quarterly financial data (unaudited) (Tables)

12 Months Ended

Dec. 31, 2023

Quarterly Financial Data [Abstract]

Summary of quarterly financial information

Summarized quarterly financial information for 2023 and 2022 is as follows:

	First Quarter		Second Quarter		Third Quarter		Fourth Quarter
	(dollars in thousands)
2023
Operating revenues	$	389,453	$	389,389	$	500,776	$	460,567
Operating margin	57,006		49,987		92,317		77,756
Net margin	24,410		18,414		27,127		(4,161)
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)