Fair Value Measurements (Narrative) (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Credit-risk-related contingent features	we have no credit-risk-related contingent features in our agreements with the financial institutions that would require us to post collateral for derivative positions in a liability position
Senior Notes Total [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Senior notes, fair value	779.5	$ 767.6
Senior notes, face amount	800.0	$ 800.0