Consolidated Balance Sheets (Parenthetical) - USD ($) $ in Millions	Jun. 30, 2016	Dec. 31, 2015
Operating Assets and Facilities ($123.2 and $123.3 related to a consolidated VIE)	$ 8,508.5	$ 8,204.0
Less: allowance for depreciation ($42.0 and $39.7 related to a consolidated VIE)	(2,580.3)	(2,505.6)
Nonrecourse ($2.3 and $6.9 related to a consolidated VIE)	$ 2.3	$ 6.9
Common stock, par value	$ 0.625	$ 0.625
Common stock, shares authorized	120,000,000	120,000,000
Common stock, shares issued	66,888,338	66,776,290
Common stock, shares outstanding	40,440,120	41,970,098
Treasury stock, shares outstanding	26,448,218	24,806,192
Variable Interest Entity, Primary Beneficiary [Member]
Operating Assets and Facilities ($123.2 and $123.3 related to a consolidated VIE)	$ 122.6	$ 122.9
Less: allowance for depreciation ($42.0 and $39.7 related to a consolidated VIE)	(42.0)	(39.7)
Nonrecourse ($2.3 and $6.9 related to a consolidated VIE)	$ 2.3	$ 6.9