Debt (Details) (USD $)	3 Months Ended	12 Months Ended	3 Months Ended	12 Months Ended	3 Months Ended	12 Months Ended			0 Months Ended			0 Months Ended			3 Months Ended				0 Months Ended									3 Months Ended											0 Months Ended	1 Months Ended	3 Months Ended						3 Months Ended
	Mar. 31, 2013	Dec. 31, 2012	Mar. 31, 2013 Minimum	Dec. 31, 2012 Minimum	Mar. 31, 2013 Maximum	Dec. 31, 2012 Maximum	Mar. 31, 2013 Mortgage notes payable	Dec. 31, 2012 Mortgage notes payable	Oct. 14, 2011 Sun Life	Mar. 31, 2013 Sun Life	Dec. 31, 2012 Sun Life	Aug. 04, 2011 Webster Bank, maturity date Aug 4, 2016	Mar. 31, 2013 Webster Bank, maturity date Aug 4, 2016	Dec. 31, 2012 Webster Bank, maturity date Aug 4, 2016	Mar. 31, 2013 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	Mar. 31, 2013 Bank of America Unsecured Credit Facility, due on September 10, 2016 Minimum	Mar. 31, 2013 Bank of America Unsecured Credit Facility, due on September 10, 2016 Maximum	Jul. 28, 2011 Union Fidelity	Mar. 31, 2013 Union Fidelity	Dec. 31, 2012 Union Fidelity	Mar. 31, 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	Mar. 31, 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	Mar. 31, 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	Mar. 31, 2013 Bank of America Unsecured Term Loan, due on September 10, 2017 Minimum	Mar. 31, 2013 Bank of America Unsecured Term Loan, due on September 10, 2017 Maximum	Mar. 31, 2013 CIGNA-1 Facility	Dec. 31, 2012 CIGNA-1 Facility	Mar. 31, 2013 CIGNA-2 Facility	Dec. 31, 2012 CIGNA-2 Facility	Mar. 31, 2013 CIGNA-3 Facility	Dec. 31, 2012 CIGNA-3 Facility	Feb. 14, 2013 Wells Fargo Bank, Unsecured Term Loan, due on February 14, 2020	Mar. 31, 2013 Wells Fargo Bank, Unsecured Term Loan, due on February 14, 2020	Mar. 31, 2013 Wells Fargo Bank, Unsecured Term Loan, due on February 14, 2020	Mar. 31, 2013 Wells Fargo Bank, Unsecured Term Loan, due on February 14, 2020 Minimum	Mar. 31, 2013 Wells Fargo Bank, Unsecured Term Loan, due on February 14, 2020 Maximum	Mar. 31, 2013 Wells Fargo Bank, CMBS Loan	Dec. 31, 2012 Wells Fargo Bank, CMBS Loan	Nov. 08, 2012 Wells Fargo Bank, CMBS Loan item	Mar. 31, 2013 Unsecured Credit Facility Minimum	Mar. 31, 2013 Unsecured Credit Facility Maximum
Debt instruments
Interest rate base	One-month LIBOR	One-month LIBOR													LIBOR													LIBOR											One-month LIBOR		LIBOR
Interest rate margin (as a percent)															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)	0.2037%	0.2087%
Remaining amount that can be drawn down																																								$ 125,000,000	$ 125,000,000
Fixed interest rate under interest rate swap (as a percent)																																								1.33%	1.33%
Principal outstanding	423,844,000	479,215,000					228,844,000	229,915,000		4,014,000	4,079,000		5,947,000	5,984,000	20,000,000	99,300,000				6,813,000	6,898,000	3,183,000	3,203,000	3,250,000	3,427,000	3,450,000	3,500,000	150,000,000	150,000,000				59,457,000	59,645,000	60,650,000	60,863,000	17,044,000	17,097,000		25,000,000	25,000,000			68,309,000	68,696,000
New or additional borrowings									4,100,000			6,200,000							7,200,000																					0	25,000,000
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													25,000,000
Unused commitment fee (as a percent)																																									0.35%
Unused fees															200,000																									56,000
Unsecured Term Loan																														150,000,000									150,000,000	150,000,000	150,000,000
Borrowing capacity after accordian																																							25,000,000	250,000,000	250,000,000
Deferred financing fees, net	5,903,000	4,704,000																																						1,400,000	1,400,000
Deferred financing fees amortization period																																									7 years
Annual fees																																									50,000
Amortization period of annual fees																																									1 year
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)			1.86%	1.86%	4.64%	4.64%
Fair value							240,533,000	242,175,000							20,000,000	99,300,000												150,000,000	150,000,000											25,000,000	25,000,000
Number of properties owned by Company's used to secure the credit facility																																														28
Remaining borrowing capacity															$ 180,000,000																		$ 0		$ 2,900,000		$ 47,900,000