Debt - Summary (Details) - USD ($)		9 Months Ended
	Dec. 31, 2014	Sep. 30, 2015	Sep. 29, 2015	Sep. 28, 2015	Feb. 20, 2015
Debt Instrument [Line Items]
Long-term debt	$ 686,347,000	$ 860,318,000
Weighted-average interest rate (as a percent)		4.09%
Notional amount		$ 300,000,000
One-Month LIBOR
Debt Instrument [Line Items]
Interest rate margin (as a percent)	0.17125%	0.193%
Unsecured line of credit
Debt Instrument [Line Items]
Long-term debt	$ 131,000,000	$ 177,750,000
Unsecured term loans | Unsecured line of credit
Debt Instrument [Line Items]
Long-term debt	150,000,000	150,000,000
Unsecured notes
Debt Instrument [Line Items]
Long-term debt	180,000,000	300,000,000
Mortgage notes
Debt Instrument [Line Items]
Long-term debt	225,347,000	232,568,000
Long-term debt, gross	225,039,000	232,011,000
Total unamortized fair market value premium	308,000	557,000
$450 Million Wells Fargo Unsecured Credit Facility | Unsecured line of credit
Debt Instrument [Line Items]
Long-term debt	131,000,000	177,750,000
Debt instrument, face amount		$ 450,000,000
Line of credit facility, maximum borrowing capacity			$ 450,000,000.0	$ 300,000,000.0
$450 Million Wells Fargo Unsecured Credit Facility | Unsecured line of credit | LIBOR
Debt Instrument [Line Items]
Interest rate margin (as a percent)		1.15%
$150 Million Wells Fargo Unsecured Term Loan A | Unsecured term loans | Unsecured line of credit
Debt Instrument [Line Items]
Long-term debt	150,000,000	$ 150,000,000
Debt instrument, face amount		$ 150,000,000
$150 Million Wells Fargo Unsecured Term Loan A | Unsecured term loans | Unsecured line of credit | LIBOR
Debt Instrument [Line Items]
Interest rate margin (as a percent)		1.65%
$150 Million Wells Fargo Unsecured Term Loan B | Unsecured term loans | Unsecured line of credit
Debt Instrument [Line Items]
Long-term debt	0	$ 0
Debt instrument, face amount		$ 150,000,000
$150 Million Wells Fargo Unsecured Term Loan B | Unsecured term loans | Unsecured line of credit | LIBOR
Debt Instrument [Line Items]
Interest rate margin (as a percent)		1.70%
$150 Million Wells Fargo Unsecured Term Loan C | Unsecured term loans | Unsecured line of credit
Debt Instrument [Line Items]
Long-term debt	0	$ 0
Debt instrument, face amount		$ 150,000,000	$ 150,000,000.0
$150 Million Wells Fargo Unsecured Term Loan C | Unsecured term loans | Unsecured line of credit | LIBOR
Debt Instrument [Line Items]
Interest rate margin (as a percent)		1.30%
$50 Million Series A Unsecured Notes | Unsecured notes
Debt Instrument [Line Items]
Long-term debt	50,000,000	$ 50,000,000
Debt instrument, face amount		$ 50,000,000
Interest rate (as a percent)		4.98%
$50 Million Series B Unsecured Notes | Unsecured notes
Debt Instrument [Line Items]
Long-term debt	50,000,000	$ 50,000,000
Debt instrument, face amount		$ 50,000,000
Interest rate (as a percent)		4.98%
$80 Million Series C Unsecured Notes | Unsecured notes
Debt Instrument [Line Items]
Long-term debt	80,000,000	$ 80,000,000
Debt instrument, face amount		$ 80,000,000
Interest rate (as a percent)		4.42%
$100 Million Series D Unsecured Notes | Unsecured notes
Debt Instrument [Line Items]
Long-term debt	0	$ 100,000,000
Debt instrument, face amount		$ 100,000,000			$ 100,000,000
Interest rate (as a percent)		4.32%			4.32%
$20 Million Series E Unsecured Notes | Unsecured notes
Debt Instrument [Line Items]
Long-term debt	0	$ 20,000,000
Debt instrument, face amount		$ 20,000,000			$ 20,000,000
Interest rate (as a percent)		4.42%			4.42%
Sun Life Assurance Company of Canada (U.S.) | Mortgage notes
Debt Instrument [Line Items]
Interest rate (as a percent)		6.05%
Long-term debt, gross	3,445,000	$ 3,285,000
Webster Bank, National Association | Mortgage notes
Debt Instrument [Line Items]
Interest rate (as a percent)		4.22%
Long-term debt, gross	5,677,000	$ 5,555,000
National Life Insurance Company | Mortgage notes
Debt Instrument [Line Items]
Interest rate (as a percent)		5.75%
Long-term debt, gross	0	$ 4,817,000
Mortgage Loan From Union Fidelity Life Insurance Company [Member] | Mortgage notes
Debt Instrument [Line Items]
Interest rate (as a percent)		5.81%
Long-term debt, gross	6,103,000	$ 5,843,000
Prepayment terms - Penalty-free pre-payable period		2 months
Principal Life Insurance Company | Mortgage notes
Debt Instrument [Line Items]
Interest rate (as a percent)		5.73%
Long-term debt, gross	0	$ 5,705,000
Webster Bank, maturity date May 29, 2017 | Mortgage notes
Debt Instrument [Line Items]
Interest rate (as a percent)		3.66%
Long-term debt, gross	3,035,000	$ 2,968,000
Webster Bank, maturity date May 31, 2017 | Mortgage notes
Debt Instrument [Line Items]
Interest rate (as a percent)		3.64%
Long-term debt, gross	3,268,000	$ 3,197,000
Wells Fargo, National Association | Mortgage notes
Debt Instrument [Line Items]
Interest rate (as a percent)		5.90%
Long-term debt, gross	4,182,000	$ 4,131,000
CIGNA-1 Facility | Mortgage notes
Debt Instrument [Line Items]
Interest rate (as a percent)		6.50%
Long-term debt, gross	58,050,000	$ 57,396,000
CIGNA-2 Facility | Mortgage notes
Debt Instrument [Line Items]
Interest rate (as a percent)		5.75%
Long-term debt, gross	59,065,000	$ 58,335,000
CIGNA-3 Facility | Mortgage notes
Debt Instrument [Line Items]
Interest rate (as a percent)		5.88%
Long-term debt, gross	16,647,000	$ 16,464,000
CIGNA Facility | Mortgage notes
Debt Instrument [Line Items]
Prepayment terms - Penalty-free pre-payable period		6 months
Wells Fargo, CMBS Loan | Mortgage notes
Debt Instrument [Line Items]
Interest rate (as a percent)		4.31%
Long-term debt, gross	$ 65,567,000	$ 64,315,000
National Life Insurance Company and Principal Life Insurance Company | Mortgage notes
Debt Instrument [Line Items]
Prepayment terms - Penalty-free pre-payable period		3 months