CONSOLIDATED STATEMENTS OF CHANGES IN STOCKHOLDERS' EQUITY - USD ($) $ in Millions	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)
Comprehensive income (loss)
Net income	393.8	0.0	0.0	387.6	0.0	6.2	0.0
Other comprehensive income, net of tax	27.1	0.0	0.0	0.0	27.0	0.1	0.0
Dividends declared on common stock	(168.8)	0.0	0.0	(168.8)	0.0	0.0	0.0
Issuance of common stock	14.2	0.0	14.2	0.0	0.0	0.0	0.0
Stock-based compensation	0.9	0.0	(1.8)	0.0	0.0	(0.8)	3.5
Contributions from noncontrolling interest partners	107.0	0.0	22.5	0.0	0.0	84.5	0.0
Distributions to noncontrolling interest partners	(2.5)	0.0	0.0	0.0	0.0	(2.5)	0.0
Deconsolidation of Enogex Holdings	(398.3)	0.0	0.0	0.5	(6.1)	(392.7)	0.0
Deferred income taxes attributable to contributions from noncontrolling interest partners	(8.7)	0.0	(8.7)	0.0	0.0	0.0	0.0
2-for-1 forward stock split	0.0	1.0	(1.0)	0.0	0.0	0.0	0.0
Balance at Dec. 31, 2013	3,037.1	2.0	1,071.6	1,991.7	(28.2)	0.0	0.0
Comprehensive income (loss)
Net income	395.8	0.0	0.0	395.8	0.0	0.0	0.0
Other comprehensive income, net of tax	(13.2)	0.0	0.0	0.0	(13.2)	0.0	0.0
Dividends declared on common stock	(189.3)	0.0	0.0	(189.3)	0.0	0.0	0.0
Issuance of common stock	13.2	0.0	13.2	0.0	0.0	0.0	0.0
Stock-based compensation	0.8	0.0	0.8	0.0	0.0	0.0	0.0
Contributions from noncontrolling interest partners	0.0
Distributions to noncontrolling interest partners	0.0
Balance at Dec. 31, 2014	3,244.4	2.0	1,085.6	2,198.2	(41.4)	0.0	0.0
Comprehensive income (loss)
Net income	271.3	0.0	0.0	271.3	0.0	0.0	0.0
Other comprehensive income, net of tax	6.3	0.0	0.0	0.0	6.3	0.0	0.0
Dividends declared on common stock	(209.7)	0.0	0.0	(209.7)	0.0	0.0	0.0
Issuance of common stock	7.2	0.0	7.2	0.0	0.0	0.0	0.0
Stock-based compensation	6.5	0.0	6.5	0.0	0.0	0.0	0.0
Contributions from noncontrolling interest partners	0.0
Distributions to noncontrolling interest partners	0.0
Balance at Dec. 31, 2015	$ 3,326.0	$ 2.0	$ 1,099.3	$ 2,259.8	$ (35.1)	$ 0.0	$ 0.0