Debt (Narrative) (Details) (USD $)	3 Months Ended		6 Months Ended					6 Months Ended			6 Months Ended
	Jun. 30, 2012	Jun. 30, 2011	Jun. 30, 2012	Jun. 30, 2011	Mar. 31, 2012	Dec. 31, 2011	Dec. 31, 2010	Jun. 30, 2012 6.75% Senior Notes Due 2013 [Member]	May 30, 2012 6.75% Senior Notes Due 2013 [Member]	May 16, 2012 6.75% Senior Notes Due 2013 [Member]	Jun. 30, 2012 Loans Payable [Member]	Jun. 30, 2012 Line of Credit [Member]	Jun. 30, 2012 Maximum [Member] 6.75% Senior Notes Due 2013 [Member]	Jun. 30, 2012 Minimum [Member] Loans Payable [Member]
Debt Instrument [Line Items]
Debt Instrument, Face Amount								$ 450,000,000			$ 1,400,000,000
Line of credit facility, maximum borrowing capacity	600,000,000		600,000,000		2,000,000,000
Loans payable	1,000,000,000		1,000,000,000
Debt instrument, maturity period											6 years
Debt instrument, discount (premium), percentage											1.00%
Debt instrument, periodic principal payment											3,500,000
Frequency of periodic payment											Quarterly
Stated interest rate								6.75%						1.25%
Basis spread on variable rate											4.50%
Tender offer, offering price per unit of principal													1.0025
Debt Instrument, Repurchased Face Amount								304,000,000
Debt instrument, repurchase amount									146,000,000	304,800,000
Net loss resulting from early retirement of debt and refinancing activities	19,042,000	250,000	19,042,000	250,000
Write off of deferred debt issuance cost								(1,100,000)				(17,300,000)
Loss on extinguishment of debt other than deferred debt issuance costs								800,000
Deferred finance costs	27,400,000		27,400,000									24,800,000
Remaining borrowing capacity	345,000,000		345,000,000
Cash and cash equivalents	$ 512,527,000	$ 82,345,000	$ 512,527,000	$ 82,345,000		$ 138,149,000	$ 93,593,000