Derivatives (Details) (USD $)	3 Months Ended			6 Months Ended
	Jun. 30, 2013		Jun. 30, 2012	Jun. 30, 2013		Jun. 30, 2012	Dec. 31, 2012
Derivative [Line Items]
Effective portion of change in fair value of derivative instruments	$ 18,332,000		$ (26,000)	$ 21,704,000		$ (60,000)
Derivative Liability, Fair Value, Gross Liability	(51,000)			(51,000)			(76,000)
Cash Flow Hedge Gain (Loss) to be Reclassified within Twelve Months				9,800,000
Derivative Instruments, Gain (Loss) Reclassified from Accumulated OCI into Income, Effective Portion, Net	2,366,000		2,366,000	4,732,000		4,729,000
Derivative Instruments, Gain (Loss) Recognized in Income, Ineffective Portion and Amount Excluded from Effectiveness Testing, Net	0		0	0		(3,000)
Assets [Member]
Derivative [Line Items]
Derivative, Notional Amount	300,000,000			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			75,000,000
Derivative, Description of Variable Rate Basis				3 Month LIBOR
Derivative, Fixed Interest Rate	2.087%			2.087%
Derivative Asset, Fair Value, Gross Asset	6,210,000			6,210,000			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			50,000,000
Derivative, Description of Variable Rate Basis				3 Month LIBOR
Derivative, Fixed Interest Rate	2.088%			2.088%
Derivative Asset, Fair Value, Gross Asset	4,130,000			4,130,000			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			75,000,000
Derivative, Description of Variable Rate Basis				3 Month LIBOR
Derivative, Fixed Interest Rate	2.479%			2.479%
Derivative Asset, Fair Value, Gross Asset	6,760,000			6,760,000			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			50,000,000
Derivative, Description of Variable Rate Basis				3 Month LIBOR
Derivative, Fixed Interest Rate	2.479%			2.479%
Derivative Asset, Fair Value, Gross Asset	4,419,000			4,419,000			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			50,000,000
Derivative, Description of Variable Rate Basis				3 Month LIBOR
Derivative, Fixed Interest Rate	2.479%			2.479%
Derivative Asset, Fair Value, Gross Asset	4,492,000			4,492,000			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			9,000,000
Derivative, Description of Variable Rate Basis				1 Month LIBOR
Derivative, Fixed Interest Rate	0.76%			0.76%
Swap [Member]
Derivative [Line Items]
Cash Flow Hedge Gain (Loss) to be Reclassified within Twelve Months				9,300,000
Unsecured Maturities [Member]
Derivative [Line Items]
2014	150,000,000	[2]		150,000,000	[2]
2015	350,000,000	[2]		350,000,000	[2]
Estimate of Fair Value Measurement [Member] | Fair Value, Measurements, Recurring [Member]
Derivative [Line Items]
Derivative Asset, Fair Value, Gross Asset	26,011,000			26,011,000			4,307,000
Estimate of Fair Value Measurement [Member] | Fair Value, Measurements, Recurring [Member] | .76% Derivative [Member]
Derivative [Line Items]
Derivative Liability, Fair Value, Gross Liability	$ (51,000)			$ (51,000)			$ (76,000)

[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.