Quarterly financial data (unaudited): (Tables)

12 Months Ended

Dec. 31, 2016

Quarterly financial data (unaudited):

Summary of quarterly financial information

		First Quarter		Second Quarter		Third Quarter		Fourth Quarter
		(dollars in thousands)
2016
Operating revenues	$	348,161	$	379,343	$	431,013	$	348,714
Operating margin		71,093		74,148		70,929		39,494
Net margin		20,598		23,277		18,630		(12,160	)
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