Asset Retirement Obligations (Narrative) (Details) $ in Millions	Dec. 31, 2015 USD ($)
asset retirement obligations [Line Items]
Letters of credit outstanding	$ 11.2
Asset retirement obligations [Member]
asset retirement obligations [Line Items]
Self-bonding obligations	485.5
Asset retirement obligations [Member]
asset retirement obligations [Line Items]
Surety bonds outstanding	$ 155.3