CONDENSED CONSOLIDATED STATEMENTS OF CHANGES IN STOCKHOLDERS' EQUITY (Unaudited) (USD $) In Millions, unless otherwise specified	Total	Common Stock	Premium on Common Stock	Retained Earnings	Accumulated Other Comprehensive Income (Loss)	Noncontrolling Interest	Treasury Stock
Balance at Dec. 31, 2011	$ 2,819.3	$ 1.0	$ 1,034.3	$ 1,574.8	$ (40.6)	$ 256.0	$ (6.2)
Changes in Stockholders' Equity
Net income	341.5	0	0	316.5	0	25.0	0
Other comprehensive income (loss), net of tax	(1.3)	0	0	0	(0.4)	(0.9)	0
Dividends declared on common stock	(116.2)	0	0	(116.2)	0	0	0
Issuance of common stock	10.9	0	10.9	0	0	0	0
Stock-based compensation and other	(8.4)	0	(11.6)	0	0	(2.9)	6.1
Contributions from noncontrolling interest partners	1.0	0	0	0	0	1.0	0
Distributions to noncontrolling interest partners	(10.3)	0	0	0	0	(10.3)	0
Balance at Sep. 30, 2012	3,036.5	1.0	1,033.6	1,775.1	(41.0)	267.9	(0.1)
Balance at Dec. 31, 2012	3,072.4	1.0	1,046.4	1,772.4	(49.1)	305.2	(3.5)
Changes in Stockholders' Equity
Net income	336.2	0	0	330.0	0	6.2	0
Other comprehensive income (loss), net of tax	3.4	0	0	0	3.3	0.1	0
Dividends declared on common stock	(124.2)	0	0	(124.2)	0	0	0
Issuance of common stock	10.8	0	10.8	0	0	0	0
Stock-based compensation and other	(1.5)	0	(4.2)	0	0	(0.8)	3.5
Contributions from noncontrolling interest partners	107.0	0	22.5	0	0	84.5	0
Distributions to noncontrolling interest partners	(2.5)	0	0	0	0	(2.5)	0
Deconsolidation of subsidiary	(398.3)	0	0	0.5	(6.1)	(392.7)	0
Deferred income taxes attributable to contributions from noncontrolling interest partners	(8.7)	0	(8.7)	0	0	0	0
2-for-1 forward stock split	0	1.0	(1.0)	0	0	0	0
Balance at Sep. 30, 2013	$ 2,994.6	$ 2.0	$ 1,065.8	$ 1,978.7	$ (51.9)	$ 0	$ 0