Quarterly financial data (unaudited) (Tables)

12 Months Ended

Dec. 31, 2021

Quarterly Financial Data [Abstract]

Summary of quarterly financial information

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)

The negative net margins in the fourth quarter of 2020 were due to reductions to revenue requirements in order to achieve, but not exceed, the targeted margins for interest ratio of 1.14.