Summary of Significant Accounting Policies Related Party Transactions (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012	Dec. 31, 2012
Dividends declared on common stock	$ (40.0)
Operating Costs Charged [Member] | Og and E [Member]
Related Party Transaction, Selling, General and Administrative Expenses from Transactions with Related Party	30.4	32.7
Og and E [Member] | Enogex [Member]
Revenue from Related Parties	1.8	2.8
Og and E [Member] | Natural Gas Transportation [Member] | Enogex [Member]
Related Party Transaction, Expenses from Transactions with Related Party	8.7	8.7
Og and E [Member] | Natural Gas Purchases [Member] | Enogex [Member]
Related Party Transaction, Purchases from Related Party	5.5	3.6
Related Party Transaction, Due from (to) Related Party	2.3		1.0
Og and E [Member] | Natural Gas Storage [Member] | Enogex [Member]
Related Party Transaction, Expenses from Transactions with Related Party	3.2	3.2
Physical [Member] | Derivative Purchases [Member] | Designated as Hedging Instrument [Member] | Natural Gas [Member]
Derivative, Nonmonetary Notional Amount	50,000
Retained Earnings
Dividends declared on common stock	$ (40.0)