Consolidated Balance Sheets (Parenthetical) (USD $)	6 Months Ended	12 Months Ended
	Jun. 30, 2013	Dec. 31, 2012
Common stock, par value	$ 0.625	$ 0.625
Common stock, shares authorized	120,000,000	120,000,000
Common stock, shares issued	66,335,153	66,021,444
Common stock, shares outstanding	46,217,033	46,898,924
Treasury stock, shares outstanding	20,118,120	19,122,520
VIE operating assets and facilities	$ 7,198,300,000	$ 6,855,200,000
VIE depreciation allowance	(2,239,800,000)	(2,200,800,000)
VIE nonrecourse debt	103,000,000	130,600,000
Variable Interest Entity, Primary Beneficiary [Member]
VIE operating assets and facilities	123,200,000	123,100,000
VIE depreciation allowance	(27,400,000)	(24,700,000)
VIE nonrecourse debt	30,300,000	35,100,000
Series A and B $2.50 Cumulative Convertible Preferred Stock [Member]
Preferred stock, par value	$ 1.00	$ 1
Preferred stock, shares authorized	5,000,000	5,000,000
Preferred stock, shares issued	0	15,567
Preferred stock, shares outstanding	0	15,567
Preferred stock, aggregate liquidation preference	$ 0	$ 0.9
Cumulative annual cash dividend per share on preferred stock	$ 2.5	$ 2.5