Quarterly financial data (unaudited): (Tables)

12 Months Ended

Dec. 31, 2015

Quarterly financial data (unaudited):

Summary of quarterly financial information

		First Quarter		Second Quarter		Third Quarter		Fourth Quarter
		(dollars in thousands)
2015
Operating revenues	$	339,778	$	343,741	$	368,664	$	297,642
Operating margin		68,260		63,494		69,287		56,417
Net margin		15,369		10,852		15,908		6,212
2014
Operating revenues	$	367,300	$	355,983	$	369,405	$	315,475
Operating margin		69,857		69,600		70,685		49,255
Net margin		19,223		17,196		14,453		(4,237	)