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Income Tax Matters, NOL (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended			3 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010	Mar. 31, 2011 State of Illinois
Net Operating Loss Carryforwards [Line Items]
Net operating loss carryforwards	$ 786.8
NOL carryforwards related to the excess tax benefits from vesting of employee restricted stock	1.7
AMT credit carryforwards	29.8
(Decrease) Increase in valuation allowance	$ (0.1)	$ (1.3)	$ 2.1	$ 0.8

The amount of the tax credit carryforward, before tax effects, available to reduce future taxable income under enacted tax laws.

The amount of the change in the period in the valuation allowance for a specified deferred tax asset.