Fair Value Measurements (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended			6 Months Ended		12 Months Ended							12 Months Ended
	Jun. 30, 2013	Mar. 31, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012	Dec. 31, 2012	Jun. 30, 2013 Quoted Prices in Active Markets for Identical Assets (Level 1) [Member]	Dec. 31, 2012 Quoted Prices in Active Markets for Identical Assets (Level 1) [Member]	Jun. 30, 2013 Significant Other Observable Inputs (Level 2) [Member]	Dec. 31, 2012 Significant Other Observable Inputs (Level 2) [Member]	Jun. 30, 2013 Fair Value, Inputs, Level 3 [Member]	Dec. 31, 2012 Fair Value, Inputs, Level 3 [Member]	Dec. 31, 2012 Cost-method Investments [Member]	Jun. 30, 2013 Cost-method Investments [Member]	Dec. 31, 2011 Cost-method Investments [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Impairment of Real Estate	$ 21,038		$ 0	$ 21,038	$ 0								$ 2,400
Assets:
Fair value of available-for-sale securities	16,304			16,304		27,679	16,304	16,556	0	0	0	11,123
Cost Method Investments, Fair Value Disclosure						2,475		0		0		2,475	2,475	0	2,475
Derivative Asset, Number of Instruments Held	1			1		1
Liabilities:
Interest rate swaps	4,528			4,528		5,805			4,528	5,805
Derivative Liability, Number of Instruments Held	4			4		4
Long-term Debt, Fair Value	$ 4,817,491			$ 4,817,491		$ 5,058,411
Available-for-sale Securities, Debt Maturities, Date		Oct. 01, 2028
Assumed Reduction In Cash Flows Used To Calculate Fair Value Of Cost Method Investment						0% to 10%
Assumed Capitalization Rate Range in Determining Fair Value						10% to 12%
Discount Rate Used To Calculate Fair Value Of Privately Held Debt And Equity Securities						20.00%