Postretirement Benefits (Details 13) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Schedule of Defined Contribution Plans Disclosures [Line Items]
Employer matching contribution to plan	$ 10	$ 9	$ 9
Class A common stock held by plan participants (in shares)	0.2	0.2
100% employer matching contribution
Schedule of Defined Contribution Plans Disclosures [Line Items]
Employer matching contribution of employee contributions (as a percent)	100.00%
100% employer matching contribution | Maximum
Schedule of Defined Contribution Plans Disclosures [Line Items]
Percentage of eligible employee compensation, matched by employer (as a percent)	3.00%
50% employer matching contribution
Schedule of Defined Contribution Plans Disclosures [Line Items]
Employer matching contribution of employee contributions (as a percent)	50.00%
50% employer matching contribution | Maximum
Schedule of Defined Contribution Plans Disclosures [Line Items]
Percentage of eligible employee compensation, matched by employer (as a percent)	5.00%
50% employer matching contribution | Minimum
Schedule of Defined Contribution Plans Disclosures [Line Items]
Percentage of eligible employee compensation, matched by employer (as a percent)	3.00%