CONSOLIDATED STATEMENTS OF CHANGES IN STOCKHOLDERS' EQUITY (USD $) In Millions, unless otherwise specified	Total	Common Stock	Premium on Common Stock	Retained Earnings	Accumulated Other Comprehensive Income (Loss)	Noncontrolling Interest [Member]	Treasury Stock
Balance at Dec. 31, 2008	$ 1,914.0	$ 0.9	$ 802.0	$ 1,107.6	$ (13.7)	$ 17.2	$ 0
Comprehensive income (loss)
Net income	261.1	0	0	258.3	0	2.8	0
Other comprehensive income (loss), net of tax	(61.0)	0	0	0	(61.0)	0	0
Comprehensive income (loss)	200.1	0	0	258.3	(61.0)	2.8	0
Dividends declared on common stock	(138.1)	0	0	(138.1)	0	0	0
Issuance of common stock	79.6	0.1	79.5	0	0	0	0
Stock-based compensation	5.2	0	5.2	0	0	0	0
Contributions from noncontrolling interest partners	0
Distributions to noncontrolling interest partners	0
Purchase of treasury stock	0
Balance at Dec. 31, 2009	2,060.8	1.0	886.7	1,227.8	(74.7)	20.0	0
Comprehensive income (loss)
Net income	300.4	0	0	295.3	0	5.1	0
Other comprehensive income (loss), net of tax	8.7	0	0	0	14.5	(5.8)	0
Comprehensive income (loss)	309.1	0	0	295.3	14.5	(0.7)	0
Dividends declared on common stock	(142.5)	0	0	(142.5)	0	0	0
Issuance of common stock	17.0	0	17.0	0	0	0	0
Stock-based compensation	10.4	0	10.4	0	0	0	0
Contributions from noncontrolling interest partners	183.2	0	88.1	0	0	95.1	0
Distributions to noncontrolling interest partners	(4.0)	0	0	0	0	(4.0)	0
Deferred income taxes attributable to contributions from noncontrolling interest partners	(34.0)	0	(34.0)	0	0	0	0
Purchase of treasury stock	0
Balance at Dec. 31, 2010	2,400.0	1.0	968.2	1,380.6	(60.2)	110.4	0
Comprehensive income (loss)
Net income	363.6	0	0	342.9	0	20.7	0
Other comprehensive income (loss), net of tax	19.9	0	0	0	19.6	0.3	0
Comprehensive income (loss)	383.5	0	0	342.9	19.6	21.0	0
Dividends declared on common stock	(148.7)	0	0	(148.7)	0	0	0
Issuance of common stock	14.8	0	14.8	0	0	0	0
Stock-based compensation	5.8	0	5.8	0	0	0	0
Contributions from noncontrolling interest partners	216.4	0	74.4	0	0	142.0	0
Distributions to noncontrolling interest partners	(17.4)	0	0	0	0	(17.4)	0
Deferred income taxes attributable to contributions from noncontrolling interest partners	(28.9)	0	28.9	0	0	0	0
Purchase of treasury stock	(6.2)	0	0	0	0	0	(6.2)
Balance at Dec. 31, 2011	$ 2,819.3	$ 1.0	$ 1,034.3	$ 1,574.8	$ (40.6)	$ 256.0	$ (6.2)