Quarterly Results (Unaudited) (Tables)

12 Months Ended

Dec. 31, 2013

Quarterly Financial Data [Abstract]

Schedule of Quarterly Financial Information [Table Text Block]

	1st Quarter		2nd Quarter		3rd Quarter		4th Quarter
	(in thousands, except per share amounts)
	2013
Operating revenues	$	733,649	$	862,041	$	868,366	$	907,089
Operating income		251,960		325,317		309,229		317,117
Net income		127,515		245,631		185,867		144,490
Earnings per share - Basic		0.36		0.70		0.53		0.41
Earnings per share - Diluted		0.36		0.70		0.53		0.41
	2012
Operating revenues	$	654,835	$	606,076	$	691,727	$	777,355
Operating income (1)		184,326		(646,046)		(65,959)		(572,947)
Net income		107,704		(405,132)		(54,053)		(355,583)
Earnings per share - Basic		0.31		(1.16)		(0.16)		(1.02)
Earnings per share - Diluted		0.31		(1.16)		(0.16)		(1.02)
(1) 2012 Operating income includes an impairment charge for the full year of $1,939.7 million.