Debt And Credit Agreements (Narrative) (Details) (USD $) In Millions, unless otherwise specified	9 Months Ended
	Sep. 30, 2012	Apr. 23, 2012
Line of Credit Facility [Line Items]
Line of credit facility, covenant terms, maximum debt to EBITDA ratio	3.0
Line of credit facility, covenant terms, minimum adjusted EBIT-to-interest expense ratio	2.5
Line of credit facility, covenant terms, maximum aggregate distributions	$ 100
Line of credit facility, covenant terms, percentage of distributions to net income	50.00%
Line of credit facility, covenant terms, minimum shareholders' equity	320
Line of credit facility, covenant terms, percentage of shareholders' equity to net income	50.00%
Unsecured Revolving Credit Facility [Member]
Line of Credit Facility [Line Items]
Line of credit facility, maximum borrowing capacity	350
Line of credit facility, term	5 years
Line of credit facility, option to increase additional amount	75
Amount borrowed under the Credit Agreement		102
Previous Unsecured Revolving Credit Facility [Member]
Line of Credit Facility [Line Items]
Line of credit facility, maximum borrowing capacity	$ 300
Line of credit facility, term	4 years