Debt (Details) (USD $)							1 Months Ended		1 Months Ended
	Jun. 30, 2013 Facility	Nov. 30, 2012 Facility	Mar. 31, 2011 Facility	Jun. 30, 2013 Corporate Revolver	Apr. 30, 2013 Corporate Revolver	Dec. 31, 2012 Corporate Revolver	Jul. 31, 2013 Revolving Letter of Credit Facility Subsequent event	Jul. 29, 2013 Revolving Letter of Credit Facility Subsequent event item	Jul. 31, 2013 Revolving Letter of Credit Facility Minimum Subsequent event
Debt
Available amount under facility		$ 1,500,000,000	$ 2,000,000,000		$ 300,000,000	$ 260,000,000	$ 100,000,000
Amount outstanding	900,000,000			0		0		27,000,000
Undrawn availability	399,100,000			300,000,000		260,000,000
Scheduled maturities of debt during the five year period and thereafter
2014	34,759,000
2015	325,235,000
2016	35,272,000
2017	393,623,000
Thereafter	111,111,000
Additional commitments							$ 50,000,000
Cash collateral maintained as a percentage of outstanding letters of credit									75.00%
Cash collateral required as a percentage of outstanding letters of credit under breach of certain financial covenants							100.00%
Letter of credit fee (as a percent)							0.50%
Number of letters of credit								3