Mortgages and Notes Payable (Details) (USD $)	12 Months Ended																	12 Months Ended
	Dec. 31, 2013		Dec. 31, 2012		Dec. 31, 2011	Dec. 31, 2013 5.45% (5.12% effective rate) Mortgage Loan due 2014 [Member]	Dec. 31, 2013 5.68% and 6.48% Mortgage Loans Due 2013 [Member] loans	Dec. 31, 2013 Consolidated Affiliate 5.74% to 5.89% Mortgage Loans Due 2014 [Member] loans	Dec. 31, 2012 5.85% (5.88% effective rate) Notes due 2017 [Member]	Dec. 31, 2012 3.625% (3.752% Effective Rate) Notes Due 2023 [Member]	Dec. 31, 2013 Variable Rate Term Loan due 2016 [Member]	Dec. 31, 2012 Variable Rate Term Loan due 2016 [Member] participants	Dec. 31, 2011 Variable Rate Term Loan due 2016 [Member]	Dec. 31, 2013 Variable Rate Term Loan (1) due 2019 [Member]	Dec. 31, 2012 Variable Rate Term Loan (2) due 2019 [Member]	Dec. 31, 2013 Revolving Credit Facility due 2018 [Member] extensions	Jan. 31, 2014 Revolving Credit Facility due 2018 [Member]	Dec. 31, 2011 Variable Rate Term Loans due 2011 [Member]	Dec. 31, 2012 Variable Rate Construction Loan due 2012 [Member]	Dec. 31, 2012 6.03% Mortgage Loan due 2013 [Member]	Dec. 31, 2012 Variable Rate Term Loan due 2018 [Member] participants	Dec. 31, 2011 7.05% Mortgage Loan due 2012 [Member]	Dec. 31, 2012 Consolidated Affiliate 6.06% Mortgage Loan due 2012 [Member]	Dec. 31, 2013 Secured indebtedness [Member]		Dec. 31, 2012 Secured indebtedness [Member]		Dec. 31, 2013 Secured indebtedness [Member] 5.68% Mortgage Loan due 2013 [Member]	Dec. 31, 2012 Secured indebtedness [Member] 5.68% Mortgage Loan due 2013 [Member]	Dec. 31, 2013 Secured indebtedness [Member] 5.45% (5.12% effective rate) Mortgage Loan due 2014 [Member]		Dec. 31, 2012 Secured indebtedness [Member] 5.45% (5.12% effective rate) Mortgage Loan due 2014 [Member]		Dec. 31, 2013 Secured indebtedness [Member] 5.21% (3.11% effective rate) Mortgage Loan due 2014 [Member]		Dec. 31, 2012 Secured indebtedness [Member] 5.21% (3.11% effective rate) Mortgage Loan due 2014 [Member]		Dec. 31, 2013 Secured indebtedness [Member] 5.17% (6.43% effective rate) Mortage Loan due 2015 [Member]		Dec. 31, 2012 Secured indebtedness [Member] 5.17% (6.43% effective rate) Mortage Loan due 2015 [Member]		Dec. 31, 2013 Secured indebtedness [Member] 3.50% (3.34% effective rate) Mortgage Loan due 2015 [Member]		Dec. 31, 2012 Secured indebtedness [Member] 3.50% (3.34% effective rate) Mortgage Loan due 2015 [Member]		Dec. 31, 2013 Secured indebtedness [Member] 6.88% Mortgage Loan due 2016 [Member]	Dec. 31, 2012 Secured indebtedness [Member] 6.88% Mortgage Loan due 2016 [Member]	Dec. 31, 2013 Secured indebtedness [Member] 7.50% Mortgage Loan due 2016 [Member]	Dec. 31, 2012 Secured indebtedness [Member] 7.50% Mortgage Loan due 2016 [Member]	Dec. 31, 2013 Secured indebtedness [Member] 5.10%(4.22% effective rate) Mortgage Loan due 2017 [Member]		Dec. 31, 2012 Secured indebtedness [Member] 5.10%(4.22% effective rate) Mortgage Loan due 2017 [Member]		Dec. 31, 2013 Secured indebtedness [Member] 5.74% to 9.00% Mortgage Loans due between 2012 and 2016 [Member]		Dec. 31, 2012 Secured indebtedness [Member] 5.74% to 9.00% Mortgage Loans due between 2012 and 2016 [Member]		Dec. 31, 2013 Unsecured indebtedness [Member]	Dec. 31, 2012 Unsecured indebtedness [Member]	Dec. 31, 2013 Unsecured indebtedness [Member] 5.85% (5.88% effective rate) Notes due 2017 [Member]		Dec. 31, 2012 Unsecured indebtedness [Member] 5.85% (5.88% effective rate) Notes due 2017 [Member]		Dec. 31, 2013 Unsecured indebtedness [Member] 7.50% Notes due 2018 [Member]	Dec. 31, 2012 Unsecured indebtedness [Member] 7.50% Notes due 2018 [Member]	Dec. 31, 2013 Unsecured indebtedness [Member] 3.625% (3.752% Effective Rate) Notes Due 2023 [Member]		Dec. 31, 2012 Unsecured indebtedness [Member] 3.625% (3.752% Effective Rate) Notes Due 2023 [Member]		Dec. 31, 2013 Unsecured indebtedness [Member] Variable Rate Term Loan due 2016 [Member]	Dec. 31, 2012 Unsecured indebtedness [Member] Variable Rate Term Loan due 2016 [Member]	Dec. 31, 2013 Unsecured indebtedness [Member] Variable Rate Term Loan (1) due 2019 [Member]		Dec. 31, 2012 Unsecured indebtedness [Member] Variable Rate Term Loan (1) due 2019 [Member]		Dec. 31, 2013 Unsecured indebtedness [Member] Variable Rate Term Loan (2) due 2019 [Member]		Dec. 31, 2012 Unsecured indebtedness [Member] Variable Rate Term Loan (2) due 2019 [Member]		Dec. 31, 2013 Unsecured indebtedness [Member] Revolving Credit Facility due 2018 [Member]		Dec. 31, 2012 Unsecured indebtedness [Member] Revolving Credit Facility due 2018 [Member]
Debt Instrument [Line Items]
Mortgages and notes payable	$ 1,956,299,000		$ 1,859,162,000																					$ 488,664,000	[1]	$ 549,607,000	[1]	$ 0	$ 107,289,000	$ 0	[2]	$ 67,604,000	[2]	$ 125,247,000	[3]	$ 0	[3]	$ 39,609,000	[4]	$ 39,805,000	[4]	$ 37,340,000	[5]	$ 0	[5]	$ 109,167,000	$ 110,671,000	$ 45,103,000	$ 45,662,000	$ 118,126,000	[6]	$ 120,924,000	[6]	$ 14,072,000	[7],[8]	$ 57,652,000	[7],[8]	$ 1,467,635,000	$ 1,309,555,000	$ 379,311,000	[9]	$ 379,194,000	[9]	$ 200,000,000	$ 200,000,000	$ 247,624,000	[10]	$ 247,361,000	[10]	$ 0	$ 35,000,000	$ 200,000,000	[11]	$ 200,000,000	[11]	$ 225,000,000	[12]	$ 225,000,000	[12]	$ 215,700,000	[13]	$ 23,000,000	[13]
Scheduled future principal payments [Abstract]
2014	138,763,000
2015	81,232,000
2016	158,218,000
2017	488,711,000
2018	415,437,000
Thereafter	673,938,000
Total scheduled future principal payments	1,956,299,000		1,859,162,000																					488,664,000	[1]	549,607,000	[1]	0	107,289,000	0	[2]	67,604,000	[2]	125,247,000	[3]	0	[3]	39,609,000	[4]	39,805,000	[4]	37,340,000	[5]	0	[5]	109,167,000	110,671,000	45,103,000	45,662,000	118,126,000	[6]	120,924,000	[6]	14,072,000	[7],[8]	57,652,000	[7],[8]	1,467,635,000	1,309,555,000	379,311,000	[9]	379,194,000	[9]	200,000,000	200,000,000	247,624,000	[10]	247,361,000	[10]	0	35,000,000	200,000,000	[11]	200,000,000	[11]	225,000,000	[12]	225,000,000	[12]	215,700,000	[13]	23,000,000	[13]
Aggregate undepreciated book value of secured real estate assets																								801,700,000
Unamortized premium																																200,000		700,000								100,000								3,600,000		4,600,000		300,000		500,000
Unamortized discount																																						800,000		1,200,000																				400,000		500,000				2,400,000		2,600,000
Mortgage debt related to consolidated affiliate	189,432,000	[14]	370,393,000	[14]																																																				33,100,000
Maximum borrowing capacity on revolving credit facility																475,000,000
Maturity date on revolving credit facility																Jan. 01, 2018
Additional borrowing capacity on revolving credit facility																75,000,000
Number of additional maturity extensions																2
Term of optional extensions																6 months
Interest rate																LIBOR plus 110 basis points
Annual facility fee (in hundredths)																0.20%
Amount outstanding on revolving credit facility																215,700,000	223,000,000
Outstanding letters of credit on revolving credit facility																100,000	100,000
Unused borrowing capacity on revolving credit facility																259,200,000	251,900,000
Repayment of loans at maturity																			52,100,000
Early repayment of debt							114,700,000	32,300,000			35,000,000.0							10,000,000		123,000,000		184,200,000
Principal amount of debt						67,500,000				250,000,000			200,000,000	200,000,000	225,000,000						200,000,000		20,800,000
Term of debt instrument (in years)													5 years	5 years	7 years			3 years			5 years
Number of debt instruments repaid							2	4
Scheduled maturity date						Jan. 01, 2014	Dec. 01, 2013	Jan. 01, 2014	Mar. 15, 2017	Jan. 15, 2023		Feb. 01, 2016		Jan. 01, 2019				Feb. 01, 2012		Mar. 01, 2013	Jan. 01, 2018	Jan. 01, 2012	Oct. 01, 2012
Interest rate basis													LIBOR plus basis points	LIBOR plus basis points	LIBOR plus basis points						LIBOR plus basis points
Interest rate, basis spread						5.12%	5.75%	5.79%	5.85%	3.625%			2.20%	1.20%	1.90%	1.10%		3.90%	1.07%	6.03%	1.65%	7.05%	6.06%
Current interest rate (in hundredths)																																																																								1.37%				3.43%				1.27%
Gross book value of unencumbered real estate assets	147,000,000
Annual repayments through principal amortization	11,700,000
Purchase price of debt (in hundredths)									107.50%	98.94%
Effective percentage rate (in hundredths)										3.752%
Original issuance discount										2,700,000
Deferred financing fees										2,100,000				400,000							900,000
Number of participant changes												2									2
Aggregate amount related to change in participants												35,000,000									35,000,000
Date of early debt repayment for participant changes												Feb. 25, 2013
Principal amount of repurchased debt									12,100,000
Penalty incurred on early repayment of debt	(199,000)		(973,000)		(24,000)		(100,000)		1,000,000		(200,000)
Lender covenant compliance to accelerate outstanding borrowings	51.00%
Holders covenant compliance of bonds	25.00%
Uncured covenant compliance default period	60 days
Capitalized Interest [Abstract]
Capitalized Interest	$ 2,700,000		$ 1,000,000		$ 600,000

[1] The secured mortgage loans payable are collateralized by real estate assets with an aggregate undepreciated book value of $801.7 million at December 31, 2013. Our fixed rate mortgage loans generally are either locked out to prepayment for all or a portion of their term or are prepayable subject to certain conditions including prepayment penalties.

[2] Net of unamortized fair market premium of $0.2 million as of December 31, 2012. This debt was repaid in 2013.

[3] Net of unamortized fair market value premium of $0.7 million as of December 31, 2013.

[4] Net of unamortized fair market value discount of $0.8 million and $1.2 million as of December 31, 2013 and 2012, respectively.

[5] Net of unamortized fair market value premium of $0.1 million as of December 31, 2013.

[6] Net of unamortized fair market premium of $3.6 million and $4.6 million as of December 31, 2013 and 2012, respectively.

[7] Net of unamortized fair market value premium of $0.3 million and $0.5 million at December 31, 2013 and 2012, respectively.

[8] Included mortgage debt related to Markel of $33.1 million at December 31, 2012. This debt was repaid in 2013.

[9] Net of unamortized original issuance discount of $0.4 million and $0.5 million at December 31, 2013 and 2012, respectively.

[10] Net of unamortized original issuance discount of $2.4 million and $2.6 million at December 31, 2013 and 2012, respectively.

[11] The interest rate is 1.37% at December 31, 2013.

[12] As more fully described in Note 7, we entered into floating-to-fixed interest rate swaps that effectively fix LIBOR for the full amount and duration of this loan. Accordingly, the equivalent fixed rate of this loan is 3.43%.

[13] The interest rate is 1.27% at December 31, 2013.

[14] Our share of scheduled future principal payments, including amortization, due on mortgages and notes payable at December 31, 2013 is as follows:2014$11,011201599220161,062201727,082201819,397Thereafter4,880 $64,424 All of this joint venture debt is non-recourse to us except in the case of customary exceptions pertaining to such matters as misuse of funds, environmental conditions, material misrepresentations and voluntary or uncontested involuntary bankruptcy events.