Consolidated Balance Sheets (USD $) In Millions, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012
Current assets:
Cash and cash equivalents	$ 63.7	$ 72.6
Accounts receivable, net	202.4	183.6
Materials and supplies	140.4	125.6
Deferred income taxes	87.7	92.1
Other current assets	48.8	48.4
Total current assets	543.0	522.3
Investments	56.6	51.5
Restricted funds	13.1	14.2
Property and equipment (including concession assets), net	5,745.7	5,684.8
Other assets	115.1	123.1
Total assets	6,473.5	6,395.9
Current liabilities:
Debt due within one year	162.5	60.2
Accounts payable and accrued liabilities	356.9	364.6
Total current liabilities	519.4	424.8
Long-term debt	1,431.1	1,547.6
Deferred income taxes	909.4	894.2
Other noncurrent liabilities and deferred credits	126.5	128.6
Total liabilities	2,986.4	2,995.2
Commitments and contingencies	0	0
Stockholders' equity:
$.01 par, common stock, 400,000,000 shares authorized; 123,352,185 shares issued; 110,181,963 and 110,131,353 shares outstanding at March 31, 2013 and December 31, 2012, respectively	1.1	1.1
Paid-in capital	930.6	925.3
Retained earnings	2,246.6	2,166.5
Accumulated other comprehensive loss	(1.8)	(2.4)
Total stockholders’ equity	3,182.6	3,096.6
Noncontrolling interest	304.5	304.1
Total equity	3,487.1	3,400.7
Total liabilities and equity	6,473.5	6,395.9
$25 Par Preferred Stock [Member]
Stockholders' equity:
$25 par, 4% noncumulative, preferred stock, 840,000 shares authorized, 649,736 shares issued, 242,170 shares outstanding	$ 6.1	$ 6.1