Capital and Financing Transactions - Interest Rate Swaps (Details) (USD $)	3 Months Ended		6 Months Ended		0 Months Ended	3 Months Ended	0 Months Ended			0 Months Ended	6 Months Ended		0 Months Ended
	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013	Apr. 02, 2014 Interest Rate Swap	Jun. 30, 2014 Interest Rate Swap	Apr. 08, 2014 Cash Flow Hedging Designated as Hedging Instrument Interest Rate Swap	Apr. 02, 2014 Cash Flow Hedging Designated as Hedging Instrument Interest Rate Swap Swap	Jun. 12, 2013 Cash Flow Hedging Designated as Hedging Instrument Interest Rate Swap	Apr. 02, 2014 Seven-year Term Loan Unsecured Debt	Jun. 30, 2014 Unsecured Term Loan $125 million Unsecured Debt	Jun. 30, 2014 Bank of America Center, Maturity May 2018 Mortgages Designated as Hedging Instrument	Apr. 02, 2014 LIBOR Seven-year Term Loan Unsecured Debt	Apr. 02, 2014 LIBOR Maximum Seven-year Term Loan Unsecured Debt	Apr. 02, 2014 LIBOR Minimum Seven-year Term Loan Unsecured Debt
Derivative [Line Items]
Notional amount					$ 5,000,000		$ 100,000,000	$ 125,000,000	$ 120,000,000
Interest expense	17,132,000	11,162,000	32,377,000	21,491,000		121,000
Derivative fixed LIBOR rate					1.70%		2.60%	0.70%	1.60%
Derivative, Notional Amount, Terminated												33,900,000
Number of interest rate swaps								2
Debt instrument face amount											125,000,000
Term of debt instrument										7 years	5 years
Derivative variable reference rate					one-month LIBOR		one-month LIBOR	one-month LIBOR
Asset value of interest rate swaps								$ 1,700,000
Variable reference rate on debt													LIBOR
Interest rate during period on debt (in percent)													1.75%
Interest rate at period end (in percent)										4.31%
Basis spread on variable rate (in percent)														2.30%	1.75%