Quarterly financial data (unaudited)

12 Months Ended

Dec. 31, 2021

Quarterly Financial Data [Abstract]

Quarterly financial data (unaudited)

Quarterly financial data (unaudited):Summarized quarterly financial information for 2021 and 2020 is as follows:

	First Quarter		Second Quarter		Third Quarter		Fourth Quarter
	(dollars in thousands)
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
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)