Quarterly financial data (unaudited): (Tables)

12 Months Ended

Dec. 31, 2018

Quarterly financial data (unaudited):

Summary of quarterly financial information

	First		Second		Third		Fourth
	Quarter		Quarter		Quarter		Quarter
	(dollars in thousands)
2018
Operating revenues	$	373,646	$	365,921	$	384,644	$	355,902
Operating margin		69,931		60,849		54,845		39,351
Net margin		27,400		17,285		11,334		(4,820)
2017
Operating revenues	$	354,170	$	361,369	$	376,656	$	342,001
Operating margin		69,330		63,472		59,520		46,548
Net margin		21,454		15,676		11,555		2,592