Investments (Additional Information) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		12 Months Ended
	Mar. 31, 2013	Mar. 31, 2012	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2000 R S Cogen Limited Liability Company [Member]
Variable Interest Entity [Line Items]
Income (loss) from continuing operations, net of tax	$ 219	$ (50)
PPG percent ownership interest						50.00%
PPG's future commitment to purchase electricity and steam from the joint venture, per year			$ 25	$ 23	$ 23