Quarterly financial data (unaudited) (Tables)

12 Months Ended

Dec. 31, 2012

Quarterly financial data (unaudited)

Summary of quarterly financial information

		First Quarter		Second Quarter		Third Quarter		Fourth Quarter
		(dollars in thousands)
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	)
2011
Operating revenues	$	269,774	$	379,803	$	432,530	$	308,171
Operating margin		65,935		63,702		64,884		43,299
Net margin		16,110		12,718		10,408		(1,499	)