Capital and Financing Transactions, Interest Rate Swaps (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012
Derivative [Line Items]
Notional amount	$ 125,000
Fair market value liability	(15,039)	(10,860)
Fund II Assets [Member]
Derivative [Line Items]
Notional amount	34,600
Swap 1 [Member] | Accounts Payable and Other Liabilities [Member]
Derivative [Line Items]
Notional amount	12,088
Reference rate (in hundredths)	1-month LIBOR
Rate at which reference rate is fixed (in hundredths)	4.10%
Maturity date	Nov. 18, 2015
Fair market value liability	(541)	(575)
Swap 2 [Member] | Accounts Payable and Other Liabilities [Member]
Derivative [Line Items]
Notional amount	30,000
Reference rate (in hundredths)	1-month LIBOR
Rate at which reference rate is fixed (in hundredths)	2.30%
Maturity date	Feb. 01, 2016
Fair market value liability	(1,666)	0
Swap 3 [Member] | Accounts Payable and Other Liabilities [Member]
Derivative [Line Items]
Notional amount	50,000
Reference rate (in hundredths)	1-month LIBOR
Rate at which reference rate is fixed (in hundredths)	2.20%
Maturity date	Sep. 27, 2017
Fair market value liability	26	0
Swap 4 [Member] | Accounts Payable and Other Liabilities [Member]
Derivative [Line Items]
Notional amount	75,000
Reference rate (in hundredths)	1-month LIBOR
Rate at which reference rate is fixed (in hundredths)	2.20%
Maturity date	Sep. 27, 2017
Fair market value liability	39	0
Swap 5 [Member] | Accounts Payable and Other Liabilities [Member]
Derivative [Line Items]
Notional amount	33,875
Reference rate (in hundredths)	1-month LIBOR
Rate at which reference rate is fixed (in hundredths)	4.70%
Maturity date	Nov. 18, 2017
Fair market value liability	(3,113)	(2,715)
Swap 6 [Member] | Accounts Payable and Other Liabilities [Member]
Derivative [Line Items]
Notional amount	22,000
Reference rate (in hundredths)	1-month LIBOR
Rate at which reference rate is fixed (in hundredths)	4.50%
Maturity date	Jan. 25, 2018
Fair market value liability	(1,804)	(1,454)
Swap 7 [Member] | Accounts Payable and Other Liabilities [Member]
Derivative [Line Items]
Notional amount	47,500
Reference rate (in hundredths)	1-month LIBOR
Rate at which reference rate is fixed (in hundredths)	5.00%
Maturity date	Jan. 25, 2018
Fair market value liability	(1,447)	(403)
Swap 8 [Member] | Accounts Payable and Other Liabilities [Member]
Derivative [Line Items]
Notional amount	9,250
Reference rate (in hundredths)	1-month LIBOR
Rate at which reference rate is fixed (in hundredths)	5.20%
Maturity date	Sep. 30, 2018
Fair market value liability	(1,140)	(1,010)
Swap 9 [Member] | Accounts Payable and Other Liabilities [Member]
Derivative [Line Items]
Notional amount	22,500
Reference rate (in hundredths)	1-month LIBOR
Rate at which reference rate is fixed (in hundredths)	5.40%
Maturity date	Oct. 08, 2018
Fair market value liability	(2,935)	(2,641)
Swap 10 [Member] | Accounts Payable and Other Liabilities [Member]
Derivative [Line Items]
Notional amount	22,100
Reference rate (in hundredths)	1-month LIBOR
Rate at which reference rate is fixed (in hundredths)	5.00%
Maturity date	Nov. 18, 2018
Fair market value liability	$ (2,458)	$ (2,062)