Debt (Narrative) (Details) (USD $)	3 Months Ended		9 Months Ended					9 Months Ended			9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011	Mar. 31, 2012	Dec. 31, 2011	Dec. 31, 2010	Sep. 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]	Sep. 30, 2012 Loans Payable [Member]	Sep. 30, 2012 Line of Credit [Member]	Sep. 30, 2012 Maximum [Member] 6.75% Senior Notes Due 2013 [Member]	Sep. 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
Proceeds from Issuance of Long-term Debt			1,386,000,000
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													1,002.50
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		1,708,000	19,042,000	1,958,000
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	366,000,000		366,000,000
Cash and cash equivalents	$ 550,756,000	$ 158,509,000	$ 550,756,000	$ 158,509,000		$ 138,149,000	$ 93,593,000