Consolidated Balance Sheets (USD $) In Millions, unless otherwise specified	Mar. 31, 2014	Dec. 31, 2013
Current assets:
Cash and cash equivalents	$ 167.5	$ 429.5
Accounts receivable, net	195.6	198.3
Materials and supplies	131.8	121.3
Deferred income taxes	125.6	131.6
Other current assets	71.2	61.7
Total current assets	691.7	942.4
Investments	40.8	41.1
Restricted funds	0	4.2
Property and equipment (including concession assets), net	6,540.9	6,356.3
Other assets	87.6	91.4
Total assets	7,361.0	7,435.4
Current liabilities:
Debt due within one year	24.9	332.0
Commercial paper	175.0	0
Accounts payable and accrued liabilities	374.4	398.6
Total current liabilities	574.3	730.6
Long-term debt	1,857.3	1,856.9
Deferred income taxes	1,067.4	1,044.6
Other noncurrent liabilities and deferred credits	122.7	126.7
Total liabilities	3,621.7	3,758.8
Commitments and contingencies	0	0
Stockholders' equity:
$.01 par, common stock, 400,000,000 shares authorized; 123,352,185 shares issued; 110,325,979 and 110,229,229 shares outstanding at March 31, 2014 and December 31, 2013, respectively	1.1	1.1
Paid-in capital	942.2	942.5
Retained earnings	2,485.6	2,422.9
Accumulated other comprehensive loss	(2.0)	(2.0)
Total stockholders’ equity	3,433.0	3,370.6
Noncontrolling interest	306.3	306.0
Total equity	3,739.3	3,676.6
Total liabilities and equity	7,361.0	7,435.4
$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