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Postretirement Benefits (Details 5) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Postretirement Benefits
Accumulated benefit obligation	$ 363	$ 297	$ 252
Periodic benefit cost	20.0	15.0	12.0
Retiree health benefit plan
Postretirement Benefits
Accumulated benefit obligation	6	7
Periodic benefit cost	$ 0.2	$ 0.3

For defined benefit pension plans, the actuarial present value of benefits (whether vested or nonvested) attributed by the pension benefit formula to employee service rendered before a specified date and based on employee service and compensation (if applicable) before that date. The accumulated benefit obligation differs from the projected benefit obligation in that it includes no assumption about future compensation levels. For plans with flat-benefit or nonpay-related pension benefit formulas, the accumulated benefit obligation and the projected benefit obligation are the same.

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