Quarterly financial data (unaudited) (Tables)

12 Months Ended

Dec. 31, 2024

Quarterly Financial Data [Abstract]

Summary of quarterly financial information

Summarized quarterly financial information for 2024 and 2023 is as follows:

	First Quarter		Second Quarter		Third Quarter		Fourth Quarter
	(dollars in thousands)
2024
Operating revenues	$	536,314	$	564,090	$	540,670	$	540,773
Operating margin	120,231		125,140		125,545		112,580
Net margin	42,099		24,191		10,560		(6,349)
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)