Statements Of Consolidated Common Stockholders' Equity (USD $) In Millions		Total	Common Stock [Member]	Treasury Stock [Member]	Additional Paid-in Capital [Member]	Retained Earnings [Member]	Accumulated Other Comprehensive Income (Loss) [Member]	Noncontrolling Interest [Member]
Beginning Balance at Dec. 31, 2014		$ 6,175.3	$ 3.2	$ (58.9)	$ 4,787.6	$ 1,494.0	$ (50.6)	$ 0
Net Income		275.3	0	0	0	268.4	0	6.9
Other comprehensive income, net of tax		2.0	0	0	0	0	2.0	0
Allocation of AOCI to Noncontrolling Interest		0	0	0	0	0	2.0	(2.0)
Common stock dividends ($0.52 per share)		(164.9)	0	0	0	(164.9)	0	0
Treasury stock acquired		(20.1)	0	(20.1)	0	0	0	0
Noncontrolling Interest, Increase from Subsidiary Equity Issuance		1,168.4	0	0	0	0	0	1,168.4
Employee stock purchase plan		1.2	0	0	1.2	0	0	0
Long-term incentive plan		11.0	0	0	11.0	0	0	0
401(k) and profit sharing issuance		19.6	0	0	19.6	0	0	0
Dividend reinvestment plan		1.9	0	0	1.9	0	0	0
Noncontrolling Interest, Decrease from Distributions to Noncontrolling Interest Holders	[1]	0	0	0	227.1	0	0	(227.1)
Ending Balance at Mar. 31, 2015		$ 7,469.7	$ 3.2	$ (79.0)	$ 5,048.4	$ 1,597.5	$ (46.6)	$ 946.2

[1] Represents the purchase of an additional 8.4% limited partner interest in Columbia OpCo, recorded at the historical carrying value of Columbia OpCo's net assets after giving effect to the $1,168.4 million equity contribution.