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	47.5	0	0	37.1	0	10.4	0
Other comprehensive income (loss), net of tax	(2.4)	0	0	0	(1.5)	(0.9)	0
Dividends declared on common stock	(38.7)	0	0	(38.7)	0	0	0
Issuance of common stock	3.7	0	3.7	0	0	0	0
Stock-based compensation and other	(13.4)	0	(16.7)	0	0	(2.6)	5.9
Distributions to noncontrolling interest partners	(5.6)	0	0	0	0	(5.6)	0
Balance at Mar. 31, 2012	2,810.4	1.0	1,021.3	1,573.2	(42.1)	257.3	(0.3)
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	28.0	0	0	23.1	0	4.9	0
Other comprehensive income (loss), net of tax	0.9	0	0	0	0.8	0.1	0
Dividends declared on common stock	(41.4)	0	0	(41.4)	0	0	0
Issuance of common stock	3.2	0	3.2	0	0	0	0
Stock-based compensation and other	(8.3)	0	(10.9)	0	0	(0.9)	3.5
Distributions to noncontrolling interest partners	(2.5)	0	0	0	0	(2.5)	0
Balance at Mar. 31, 2013	$ 3,052.3	$ 1.0	$ 1,038.7	$ 1,754.1	$ (48.3)	$ 306.8	$ 0