Derivatives (Details) (USD $)	3 Months Ended			12 Months Ended
	Mar. 31, 2013		Mar. 31, 2012	Dec. 31, 2012
Derivative [Line Items]
Net gain on cash flow derivative instruments	$ 3,372,000		$ 34,000
Derivative Asset, Fair Value, Gross Asset	7,679,000			4,307,000
Derivative Liability, Fair Value, Gross Liability	(64,000)			(76,000)
Cash Flow Hedge Gain (Loss) to be Reclassified within Twelve Months				9,500,000
Derivative Instruments, Gain (Loss) Reclassified from Accumulated OCI into Income, Effective Portion, Net	(2,367,000)		(2,364,000)
Derivative Instruments, Gain (Loss) Recognized in Income, Ineffective Portion and Amount Excluded from Effectiveness Testing, Net	0		(3,000)
Assets [Member]
Derivative [Line Items]
Derivative, Notional Amount	300,000,000
Derivative @ 2.087% 75K [Member]
Derivative [Line Items]
Derivative, Inception Date	Apr. 15, 2014
Derivative, Maturity Date	Apr. 15, 2024
Derivative, Early Termination Date	Oct. 15, 2014	[1]
Derivative, Counterparty	JPMorgan Chase Bank, N.A.
Derivative, Notional Amount	75,000,000
Derivative, Description of Variable Rate Basis	3 Month LIBOR
Derivative, Fixed Interest Rate	2.087%
Derivative Asset, Fair Value, Gross Asset	1,732,000			1,022,000
Derivative @ 2.088% 50K [Member]
Derivative [Line Items]
Derivative, Inception Date	Apr. 15, 2014
Derivative, Maturity Date	Apr. 15, 2024
Derivative, Early Termination Date	Oct. 15, 2014	[1]
Derivative, Counterparty	Bank of America, N.A.
Derivative, Notional Amount	50,000,000
Derivative, Description of Variable Rate Basis	3 Month LIBOR
Derivative, Fixed Interest Rate	2.088%
Derivative Asset, Fair Value, Gross Asset	1,147,000			672,000
Derivative @ 2.479% 75K [Member]
Derivative [Line Items]
Derivative, Inception Date	Aug. 01, 2015
Derivative, Maturity Date	Aug. 01, 2025
Derivative, Early Termination Date	Feb. 01, 2016	[1]
Derivative, Counterparty	US Bank National Association
Derivative, Notional Amount	75,000,000
Derivative, Description of Variable Rate Basis	3 Month LIBOR
Derivative, Fixed Interest Rate	2.479%
Derivative Asset, Fair Value, Gross Asset	2,081,000			1,131,000
Derivative A @ 2.479% 50K [Member]
Derivative [Line Items]
Derivative, Inception Date	Aug. 01, 2015
Derivative, Maturity Date	Aug. 01, 2025
Derivative, Early Termination Date	Feb. 01, 2016	[1]
Derivative, Counterparty	Royal Bank of Canada
Derivative, Notional Amount	50,000,000
Derivative, Description of Variable Rate Basis	3 Month LIBOR
Derivative, Fixed Interest Rate	2.479%
Derivative Asset, Fair Value, Gross Asset	1,344,000			729,000
Derivative B @ 2.479% 50K [Member]
Derivative [Line Items]
Derivative, Inception Date	Aug. 01, 2015
Derivative, Maturity Date	Aug. 01, 2025
Derivative, Early Termination Date	Feb. 01, 2016	[1]
Derivative, Counterparty	PNC Bank, N.A.
Derivative, Notional Amount	50,000,000
Derivative, Description of Variable Rate Basis	3 Month LIBOR
Derivative, Fixed Interest Rate	2.479%
Derivative Asset, Fair Value, Gross Asset	1,375,000			753,000
.76% Derivative [Member]
Derivative [Line Items]
Derivative, Inception Date	Oct. 01, 2011
Derivative, Maturity Date	Sep. 01, 2014
Derivative, Counterparty	PNC Bank, N.A.
Derivative, Notional Amount	9,000,000
Derivative, Description of Variable Rate Basis	1 Month LIBOR
Derivative, Fixed Interest Rate	0.76%
Derivative Liability, Fair Value, Gross Liability	(64,000)			(76,000)
Unsecured Maturities [Member]
Derivative [Line Items]
2014	150,000,000	[2]
2015	$ 350,000,000	[2]

[1] Represents the date specified in the agreement for either optional or mandatory early termination which will result in cash settlement.

[2] Includes unsecured public debt and unsecured credit facilities balances outstanding as of March 31, 2013.