Derivatives (Details) (USD $)	3 Months Ended		9 Months Ended			9 Months Ended				9 Months Ended			9 Months Ended			9 Months Ended			9 Months Ended			9 Months Ended			9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012	Dec. 31, 2012	Sep. 30, 2013 Derivative @ 2.087% 75K [Member]		Dec. 31, 2012 Derivative @ 2.087% 75K [Member]	Sep. 30, 2013 Assets [Member]	Sep. 30, 2013 Derivative @ 2.087% 75K [Member]		Oct. 28, 2013 Derivative @ 2.087% 75K [Member]	Sep. 30, 2013 Derivative @ 2.088% 50K [Member]		Dec. 31, 2012 Derivative @ 2.088% 50K [Member]	Sep. 30, 2013 Derivative @ 2.479% 75K [Member]		Dec. 31, 2012 Derivative @ 2.479% 75K [Member]	Sep. 30, 2013 Derivative A @ 2.479% 50K [Member]		Dec. 31, 2012 Derivative A @ 2.479% 50K [Member]	Sep. 30, 2013 Derivative B @ 2.479% 50K [Member]		Dec. 31, 2012 Derivative B @ 2.479% 50K [Member]	Sep. 30, 2013 .76% Derivative [Member]	Sep. 30, 2013 Swap [Member]	Sep. 30, 2013 Unsecured Maturities [Member]		Sep. 30, 2013 Estimate of Fair Value Measurement [Member] Fair Value, Measurements, Recurring [Member]	Dec. 31, 2012 Estimate of Fair Value Measurement [Member] Fair Value, Measurements, Recurring [Member]	Sep. 30, 2013 Estimate of Fair Value Measurement [Member] Fair Value, Measurements, Recurring [Member] .76% Derivative [Member]	Dec. 31, 2012 Estimate of Fair Value Measurement [Member] Fair Value, Measurements, Recurring [Member] .76% Derivative [Member]
Derivative [Line Items]
2014																											$ 150,000,000	[1]
2015																											350,000,000	[1]
Effective portion of change in fair value of derivative instruments	521,000	(25,000)	22,225,000	(85,000)
Derivative, Inception Date						Apr. 15, 2014				Apr. 15, 2014			Apr. 15, 2014			Aug. 01, 2015			Aug. 01, 2015			Aug. 01, 2015			Oct. 01, 2011
Derivative, Maturity Date						Apr. 15, 2024				Apr. 15, 2024			Apr. 15, 2024			Aug. 01, 2025			Aug. 01, 2025			Aug. 01, 2025			Sep. 01, 2014
Derivative, Early Termination Date						Oct. 15, 2014	[2]			Oct. 15, 2014	[2]		Oct. 15, 2014	[2]		Feb. 01, 2016	[2]		Feb. 01, 2016	[2]		Feb. 01, 2016	[2]
Derivative, Counterparty						JPMorgan Chase Bank, N.A.				JPMorgan Chase Bank, N.A.			Bank of America, N.A.			US Bank National Association			Royal Bank of Canada			PNC Bank, N.A.			PNC Bank, N.A.
Derivative, Notional Amount						75,000,000			300,000,000			95,000,000.0	50,000,000			75,000,000			50,000,000			50,000,000			9,000,000
Derivative, Description of Variable Rate Basis						3 Month LIBOR				3 Month LIBOR			3 Month LIBOR			3 Month LIBOR			3 Month LIBOR			3 Month LIBOR			1 Month LIBOR
Derivative, Fixed Interest Rate						2.087%				2.087%		2.8671%	2.088%			2.479%			2.479%			2.479%			0.76%
Derivative Asset, Fair Value, Gross Asset						6,138,000		1,022,000					4,085,000		672,000	7,001,000		1,131,000	4,662,000		729,000	4,654,000		753,000					26,540,000	4,307,000
Derivative Liability, Fair Value, Gross Liability	(45,000)		(45,000)		(76,000)																										(45,000)	(76,000)
Cash Flow Hedge Gain (Loss) to be Reclassified within Twelve Months			9,600,000																							9,200,000
Derivative Instruments, Gain (Loss) Reclassified from Accumulated OCI into Income, Effective Portion, Net	2,366,000	2,366,000	7,098,000	7,095,000
Derivative Instruments, Gain (Loss) Recognized in Income, Ineffective Portion and Amount Excluded from Effectiveness Testing, Net	$ 0	$ 0	$ 0	$ (3,000)

[1] Includes unsecured public debt and unsecured credit facilities.

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