Debt (Narrative) (Details) (USD $)			12 Months Ended			1 Months Ended		12 Months Ended	1 Months Ended		12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2012 Unsecured Revolving Credit Facility [Member]	Dec. 31, 2012 Unsecured And Uncommitted Overnight Facility [Member] D	Dec. 31, 2012 Debt Service Guaranty [Member]	Nov. 30, 2012 3.95 % Convertible Senior Unsecured Notes [Member]	Dec. 31, 2011 3.95 % Convertible Senior Unsecured Notes [Member]	Dec. 31, 2012 8.1% Senior Unsecured Notes [Member]	Oct. 31, 2012 3.38% Senior Unsecured Notes [Member]	Aug. 29, 2011 Unsecured Term Loan [Member]	Dec. 31, 2012 Medium-term Notes [Member]
Debt Instrument [Line Items]
Credit facility effective date			Sep. 30, 2011	May 01, 2010
Maximum borrowing capacity under credit facility			$ 500,000,000	$ 99,000,000
Credit facility expiration date			The facility expires in September 2015 and provides for a one-year extension upon our request and borrowing rates that float at a margin over LIBOR plus a facility fee.
Borrowing margin over LIBOR, basis points			125
Facility fees over LIBOR, basis points			25
Bids amount			250,000,000
Option to increase credit facility, maximum total amount			700,000,000
Fixed interest rate loan period (in days)				30
Debt coverage ratio					1.4
Debt service guaranty liability	74,075,000	74,075,000
Convertible senior unsecured debt						54,100,000
Debt stated interest rate						3.95%		8.10%	3.38%
Senior unsecured notes									300,000,000
Debt maturity date					Jan. 01, 2040	Aug. 01, 2026		Sep. 15, 2019	Oct. 15, 2022	May 31, 2012
Debt effective interest rate							5.34%		3.42%
Medium-term notes, matured											180,600,000
Debt issued at discount									99.62%
Proceeds from issuance of debt									296,900,000
Term loan										200,000,000
Term loan, term (years)										1
Debt instrument interest percentage during the period											5.50%
Debt instruments collateral value	1,500,000,000	1,700,000,000
Unsecured notes payable under credit facilities	66,000,000	166,500,000
Obligations under capital leases	21,000,000	21,000,000
Fair value interest rate contracts	9,900,000
Debt premium/(discount) net	900,000
Non-cash debt	$ 10,200,000