Debt - Additional Information (Details)	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2015 USD ($) $ / shares	Sep. 30, 2014 USD ($)	Sep. 30, 2015 USD ($) $ / shares shares	Sep. 30, 2014 USD ($) shares	Dec. 31, 2010 USD ($)	Dec. 31, 2014 USD ($)
Debt Instrument [Line Items]
Mortgages and notes payable	$ 804,238,000		$ 804,238,000			$ 945,216,000
Weighted average interest rate	5.00%		5.00%			5.20%
Percent of notes required to be repurchased at the option of the holders					100.00%
Converted debt, shares issued | shares			519,664	624,103
Convertible debt cash payments	$ 529,000	$ 62,000	$ 529,000	$ 62,000
Debt satisfaction gains (charges), net	(398,000)	$ (455,000)	13,753,000	(7,946,000)
Convertible Debt [Member]
Debt Instrument [Line Items]
Amount of debt converted			3,828,000	4,205,000
Unsecured Debt [Member] | Unsecured Credit Agreement [Member]
Debt Instrument [Line Items]
Maximum borrowing capacity	905,000,000		905,000,000
Maximum borrowing capacity with lender approval	$ 1,810,000,000		$ 1,810,000,000
6% Convertible Guaranteed Note [Member]
Debt Instrument [Line Items]
Stated interest rate	6.00%		6.00%			6.00%
Face amount of debt instrument	$ 12,400,000		$ 12,400,000		$ 115,000,000	$ 16,228,000
Maturity date			Jan. 31, 2030
Conversion ratio numerator			153.8603
Conversion price (dollars per share) | $ / shares	$ 6.50		$ 6.50
Debt satisfaction gains (charges), net			$ (476,000)	(933,000)
Other Debt Obligations [Member]
Debt Instrument [Line Items]
Debt satisfaction gains (charges), net			$ 4,123,000	$ (7,013,000)
Minimum [Member]
Debt Instrument [Line Items]
Stated interest rate	2.20%		2.20%			2.20%
Maximum [Member]
Debt Instrument [Line Items]
Stated interest rate	7.80%		7.80%			8.50%