Quarterly financial data (unaudited) (Tables)

12 Months Ended

Dec. 31, 2019

Quarterly Financial Data [Abstract]

Summary of quarterly financial information

Summarized quarterly financial information for 2019 and 2018 is as follows:

	First Quarter		Second Quarter		Third Quarter		Fourth Quarter
	(dollars in thousands)
2019
Operating revenues	$	356,600	$	358,860	$	382,623	$	332,209
Operating margin	66,032		51,712		59,209		40,256
Net margin	23,596		9,383		18,286		3,196
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)