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	149.1	0	0	131.0	0	18.1	0
Other comprehensive income (loss), net of tax	(1.8)	0	0	0	(0.9)	(0.9)	0
Dividends declared on common stock	(77.4)	0	0	(77.4)	0	0	0
Issuance of common stock	7.0	0	7.0	0	0	0	0
Stock-based compensation and other	(11.0)	0	(14.1)	0	0	(3.0)	6.1
Contributions from noncontrolling interest partners	1.0	0	0	0	0	1.0	0
Distributions to noncontrolling interest partners	(8.0)	0	0	0	0	(8.0)	0
Deconsolidation of Enogex Holdings	0
Balance at Jun. 30, 2012	2,878.2	1.0	1,027.2	1,628.4	(41.5)	263.2	(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	121.0	0	0	114.8	0	6.2	0
Other comprehensive income (loss), net of tax	2.0	0	0	0	1.9	0.1	0
Dividends declared on common stock	(82.8)	0	0	(82.8)	0	0	0
Issuance of common stock	6.9	0	6.9	0	0	0	0
Stock-based compensation and other	(5.7)	0	(8.4)	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	6.1
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
Balance at Jun. 30, 2013	$ 2,811.3	$ 2.0	$ 1,057.7	$ 1,804.9	$ (53.3)	$ 0	$ 0