Debt (Details) (USD $)	9 Months Ended	12 Months Ended	9 Months Ended	12 Months Ended	9 Months Ended	12 Months Ended			0 Months Ended			0 Months Ended			3 Months Ended		9 Months Ended					0 Months Ended									9 Months Ended											3 Months Ended	7 Months Ended	9 Months Ended						9 Months Ended
	Sep. 30, 2013	Dec. 31, 2012	Sep. 30, 2013 Minimum	Dec. 31, 2012 Minimum	Sep. 30, 2013 Maximum	Dec. 31, 2012 Maximum	Sep. 30, 2013 Mortgage notes payable	Dec. 31, 2012 Mortgage notes payable	Oct. 14, 2011 Sun Life	Sep. 30, 2013 Sun Life	Dec. 31, 2012 Sun Life	Aug. 04, 2011 Webster Bank, maturity date Aug 4, 2016	Sep. 30, 2013 Webster Bank, maturity date Aug 4, 2016	Dec. 31, 2012 Webster Bank, maturity date Aug 4, 2016	Sep. 30, 2013 Bank of America Unsecured Credit Facility, due on September 10, 2016	Sep. 30, 2012 Bank of America Unsecured Credit Facility, due on September 10, 2016	Sep. 30, 2013 Bank of America Unsecured Credit Facility, due on September 10, 2016	Sep. 30, 2012 Bank of America Unsecured Credit Facility, due on September 10, 2016	Dec. 31, 2012 Bank of America Unsecured Credit Facility, due on September 10, 2016	Sep. 30, 2013 Bank of America Unsecured Credit Facility, due on September 10, 2016 Minimum	Sep. 30, 2013 Bank of America Unsecured Credit Facility, due on September 10, 2016 Maximum	Jul. 28, 2011 Union Fidelity	Sep. 30, 2013 Union Fidelity	Dec. 31, 2012 Union Fidelity	Sep. 30, 2013 Webster Bank, maturity date May 29, 2017	Dec. 31, 2012 Webster Bank, maturity date May 29, 2017	May 29, 2012 Webster Bank, maturity date May 29, 2017	Sep. 30, 2013 Webster Bank, maturity date May 31, 2017	Dec. 31, 2012 Webster Bank, maturity date May 31, 2017	May 31, 2012 Webster Bank, maturity date May 31, 2017	Sep. 30, 2013 Bank of America Unsecured Term Loan, due on September 10, 2017	Dec. 31, 2012 Bank of America Unsecured Term Loan, due on September 10, 2017	Sep. 10, 2012 Bank of America Unsecured Term Loan, due on September 10, 2017	Sep. 30, 2013 Bank of America Unsecured Term Loan, due on September 10, 2017 Minimum	Sep. 30, 2013 Bank of America Unsecured Term Loan, due on September 10, 2017 Maximum	Sep. 30, 2013 CIGNA-1 Facility	Dec. 31, 2012 CIGNA-1 Facility	Sep. 30, 2013 CIGNA-2 Facility	Dec. 31, 2012 CIGNA-2 Facility	Sep. 30, 2013 CIGNA-3 Facility	Dec. 31, 2012 CIGNA-3 Facility	Sep. 30, 2013 Wells Fargo, Unsecured Term Loan, due on February 14, 2020	Sep. 30, 2013 Wells Fargo, Unsecured Term Loan, due on February 14, 2020	Sep. 30, 2013 Wells Fargo, Unsecured Term Loan, due on February 14, 2020	Sep. 30, 2013 Wells Fargo, Unsecured Term Loan, due on February 14, 2020 Minimum	Sep. 30, 2013 Wells Fargo, Unsecured Term Loan, due on February 14, 2020 Maximum	Sep. 30, 2013 Wells Fargo, CMBS Loan	Dec. 31, 2012 Wells Fargo, CMBS Loan	Nov. 08, 2012 Wells Fargo, CMBS Loan item	Sep. 30, 2013 Unsecured Credit Facility Minimum	Sep. 30, 2013 Unsecured Credit Facility Maximum
Debt instruments
Interest rate base	One-month LIBOR	One-month LIBOR															LIBOR														LIBOR													LIBOR
Interest rate margin (as a percent)															1.65%		1.65%		1.65%	1.65%	2.25%										1.65%	1.65%		1.65%	2.25%							2.15%	2.15%	2.15%	2.15%	2.70%
Interest Rate (as a percent)										6.05%			4.22%										5.81%		3.66%			3.64%								6.50%		5.75%		5.88%							4.31%
One-month LIBOR rate (as a percent)	0.17885%	0.2087%
Principal outstanding	$ 496,686,000	$ 479,215,000					$ 226,686,000	$ 229,915,000		$ 3,884,000	$ 4,079,000		$ 5,872,000	$ 5,984,000	$ 20,000,000		$ 20,000,000		$ 99,300,000				$ 6,639,000	$ 6,898,000	$ 3,141,000	$ 3,203,000	$ 3,250,000	$ 3,383,000	$ 3,450,000	$ 3,500,000	$ 150,000,000	$ 150,000,000				$ 59,071,000	$ 59,645,000	$ 60,213,000	$ 60,863,000	$ 16,935,000	$ 17,097,000	$ 100,000,000	$ 100,000,000	$ 100,000,000			$ 67,548,000	$ 68,696,000	$ 68,800,000
New or additional borrowings									4,100,000			6,200,000										7,200,000
Unamortized fair market value premium										200,000	200,000												100,000	200,000
Portion of unsecured term loan swapped from LIBOR for fixed rate																															100,000,000													125,000,000
Unused commitment fee (as a percent)																																												0.35%
Unused fees															200,000	40,000	500,000	40,000																								100,000	200,000
Unsecured Term Loan																																	150,000,000									150,000,000	150,000,000	150,000,000
Borrowing capacity after accordion																																										250,000,000	250,000,000	250,000,000
Variable rate received under agreement																															One-month LIBOR
Consolidated leverage ratio																																																			0.60
Secured leverage ratio																																																			0.45
Unencumbered leverage ratio																																																			0.60
Secured recourse debt ratio																																																			7.50
Fixed charge ratio																																																		1.50
Discount rate at which cash flows are discounted (as a percent)	11.40%		1.58%	1.86%	5.24%	4.64%
Fair value							230,364,000	242,175,000							20,117,000		20,117,000		99,300,000												150,444,000	150,000,000										96,951,000	96,951,000	96,951,000
Number of buildings owned by Company's serving as collateral																																																	28
Remaining borrowing capacity																																				0		2,900,000		47,900,000		50,000,000	50,000,000	50,000,000
Maximum borrowing capacity															$ 180,000,000		$ 180,000,000