Quarterly Financial Data (Unaudited) (Tables)

12 Months Ended

Dec. 31, 2012

Quarterly Financial Information Disclosure [Abstract]

Quarterly results of operations

Quarterly results of operations for the year ended December 31, 2012 are summarized below:

	Three Months Ended
	March 31		June 30		September 30		December 31
	(in thousands, except per share amounts)
2012
Operating revenue	$	301,495	$	346,308	$	342,317	$	328,268
Operating income	37,244		55,132		49,681		31,933
Net income	23,419		34,591		30,884		19,566
Basic earnings per share	0.32		0.48		0.43		0.27
Diluted earnings per share	0.32		0.48		0.43		0.27
2011
Operating revenue	$	232,662	$	275,891	$	288,714	$	273,919
Operating income	26,844		34,959		44,556		38,023
Net income	7,883		16,917		27,657		23,991
Basic earnings per share	0.30		0.41		0.38		0.33
Diluted earnings per share	0.30		0.41		0.38		0.33