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	Treasury Stock
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
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 and other	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)
Comprehensive income (loss)
Net income	385.0	0	0	355.0	0	30.0	0
Other comprehensive income (loss), net of tax	(12.0)	0	0	0	(8.5)	(3.5)	0
Dividends declared on common stock	(157.4)	0	0	157.4	0	0	0
Issuance of common stock	14.3	0	14.3	0	0	0	0
Stock-based compensation and other	(2.8)	0	(8.7)	0	0	(0.2)	6.1
Contributions from noncontrolling interest partners	46.2	0	10.7	0	0	35.5	0
Distributions to noncontrolling interest partners	(12.6)	0	0	0	0	(12.6)	0
Deferred income taxes attributable to contributions from noncontrolling interest partners	(4.2)	0	(4.2)	0	0	0	0
Purchase of treasury stock	(3.4)	0	0	0	0	0	(3.4)
Balance at Dec. 31, 2012	3,072.4	1.0	1,046.4	1,772.4	(49.1)	305.2	(3.5)
Comprehensive income (loss)
Net income	393.8	0	0	387.6	0	6.2	0
Other comprehensive income (loss), net of tax	27.1	0	0	0	27.0	0.1	0
Dividends declared on common stock	(168.8)	0	0	(168.8)	0	0	0
Issuance of common stock	14.2	0	14.2	0	0	0	0
Stock-based compensation and other	0.9	0	(1.8)	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 Enogex Holdings	(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
Purchase of treasury stock	0
Balance at Dec. 31, 2013	$ 3,037.1	$ 2.0	$ 1,071.6	$ 1,991.7	$ (28.2)	$ 0	$ 0