Quarterly financial data (unaudited):

12 Months Ended

Dec. 31, 2016

Quarterly financial data (unaudited):

Quarterly financial data (unaudited):

14. Quarterly financial data (unaudited):

    Summarized quarterly financial information for 2016 and 2015 is as follows:

		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

    The negative net margins in the fourth quarter of 2016 and the decrease in net margin in the fourth quarter of 2015 were due to reductions to revenue requirements in order to achieve the targeted margins for interest ratio of 1.14.