Summary of Significant Accounting Policies (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended	6 Months Ended
	Mar. 31, 2011	Jun. 30, 2011	Jun. 30, 2010
Description of Business
Number of joint-venture surface mines		2
Deferred taxes on certain indefinite-lived intangible assets	$ 26	$ 27	$ (2)
Montana
Description of Business
Number of joint-venture surface mines		1
Percentage of ownership of common stock in a joint-venture (as a percent)		50.00%
Wyoming
Description of Business
Number of joint-venture surface mines		1
Percentage of ownership of common stock in a joint-venture (as a percent)		50.00%