Consolidated Balance Sheets (Parenthetical) (USD $)	3 Months Ended	12 Months Ended
	Mar. 31, 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	66,256,737	66,021,444
Common stock, shares outstanding	46,765,605	46,898,924
Treasury stock, shares outstanding	19,491,132	19,122,520
VIE operating assets and facilities	$ 6,918,200,000	$ 6,855,200,000
VIE depreciation allowance	2,224,700,000	2,200,800,000
VIE nonrecourse debt	127,900,000	130,600,000
Variable Interest Entity, Primary Beneficiary [Member]
VIE operating assets and facilities	123,100,000	123,100,000
VIE depreciation allowance	(26,100,000)	(24,700,000)
VIE nonrecourse debt	32,800,000	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	15,467	15,567
Preferred stock, shares outstanding	15,467	15,567
Preferred stock, aggregate liquidation preference	$ 0.9	$ 0.9
Cumulative annual cash dividend per share on preferred stock	$ 2.5	$ 2.5