Income Taxes Income taxes (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended	12 Months Ended
	Mar. 31, 2011	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2012
Operating Loss Carryforwards [Line Items]
Tax Credit Carryforward, Federal				$ 41
Tax Credit Carryforward, State				14
Deferred Tax Assets, Valuation Allowance				0
Valuation Allowance, Deferred Tax Asset, Change in Amount		(2)
Unrecognized Tax Benefits		0	2	0
Unrecognized Tax Benefits, Interest on Income Taxes Expense			1
Unrecognized Tax Benefits, Income Tax Penalties Expense			0
Unrecognized Tax Benefits, Decreases Resulting from Current Period Tax Positions	$ 2