Quarterly financial data (unaudited) (Tables)

12 Months Ended

Dec. 31, 2013

Quarterly financial data (unaudited)

Summary of quarterly financial information

		First Quarter		Second Quarter		Third Quarter		Fourth Quarter
		(dollars in thousands)
2013
Operating revenues	$	305,914	$	324,349	$	349,725	$	265,388
Operating margin		67,554		69,795		70,266		23,909
Net margin		22,024		24,502		20,104		(25,150	)
2012
Operating revenues	$	319,224	$	347,703	$	377,396	$	279,787
Operating margin		62,056		63,919		61,051		34,807
Net margin		13,520		10,878		23,743		(8,821	)