Asset Retirement Obligations (Narrative) (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012
Minimum [Member]
Future cash flows to settle asset retirement obligations	$ 275.0
Number of years to settle retirement obligations	1 year
Maximum [Member]
Future cash flows to settle asset retirement obligations	$ 285.0
Number of years to settle retirement obligations	45 years