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, 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, 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	(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
Balance at Dec. 31, 2013	3,037.1	2.0	1,071.6	1,991.7	(28.2)	0	0
Changes in Stockholders' Equity
Net income	150.1	0	0	150.1	0	0	0
Other comprehensive income, net of tax	0.6	0	0	0	0.6	0	0
Dividends declared on common stock	(89.7)	0	0	(89.7)	0	0	0
Issuance of common stock	6.7	0	6.7	0	0	0	0
Stock-based compensation and other	(5.0)	0	(5.0)	0	0	0	0
Contributions from noncontrolling interest partners	0
Distributions to noncontrolling interest partners	0
Balance at Jun. 30, 2014	$ 3,099.8	$ 2.0	$ 1,073.3	$ 2,052.1	$ (27.6)	$ 0	$ 0