Debt - Narrative (Details)					3 Months Ended		9 Months Ended
	Sep. 29, 2015 USD ($) advance	Jun. 25, 2015 USD ($)	Feb. 20, 2015 USD ($)	Jan. 22, 2015 USD ($)	Sep. 30, 2015 USD ($)	Sep. 30, 2014 USD ($)	Sep. 30, 2015 USD ($)	Sep. 30, 2014 USD ($)	Sep. 28, 2015 USD ($)	Dec. 31, 2014 USD ($)
Debt Instrument [Line Items]
Remaining borrowing capacity					$ 572,300,000		$ 572,300,000
Assumption of mortgage notes payable							22,343,000	$ 0
Deferred financing fees, net					9,700,000		9,700,000			$ 7,800,000
Amortization of financing costs					$ 400,000	$ 400,000	$ 1,100,000	$ 1,100,000
Conyers, GA
Debt Instrument [Line Items]
Assumption of mortgage notes payable	$ 5,700,000
Mortgage notes | Burlington, NJ
Debt Instrument [Line Items]
Assumption of mortgage notes payable				$ 11,800,000
Mortgage notes | National Life Insurance Company
Debt Instrument [Line Items]
Interest rate (as a percent)					5.75%		5.75%
Mortgage notes | National Life Insurance Company | Charlotte, NC
Debt Instrument [Line Items]
Assumption of mortgage notes payable		$ 4,900,000
Interest rate (as a percent)		5.75%
Fair value premium		$ 100,000
Discount rate at which cash flows are discounted (as a percent)		3.05%
Mortgage notes | Principal Life Insurance Company
Debt Instrument [Line Items]
Interest rate (as a percent)					5.73%		5.73%
Mortgage notes | Principal Life Insurance Company | Conyers, GA
Debt Instrument [Line Items]
Assumption of mortgage notes payable	$ 5,700,000
Interest rate (as a percent)	5.73%
Fair value premium	$ 300,000
Discount rate at which cash flows are discounted (as a percent)	2.64%
Unsecured notes | $100 Million Series D Unsecured Notes
Debt Instrument [Line Items]
Debt instrument, face amount			$ 100,000,000		$ 100,000,000		$ 100,000,000
Interest rate (as a percent)			4.32%		4.32%		4.32%
Debt instrument, term			10 years
Unsecured notes | $20 Million Series E Unsecured Notes
Debt Instrument [Line Items]
Debt instrument, face amount			$ 20,000,000		$ 20,000,000		$ 20,000,000
Interest rate (as a percent)			4.42%		4.42%		4.42%
Debt instrument, term			12 years
Unsecured line of credit | $450 Million Wells Fargo Unsecured Credit Facility
Debt Instrument [Line Items]
Debt instrument, face amount					$ 450,000,000		$ 450,000,000
Line of credit facility, maximum borrowing capacity	$ 450,000,000.0								$ 300,000,000.0
Line of credit facility, accordion feature, increase limit	800,000,000.0
Deferred financing fees	1,000,000
Unsecured line of credit | $450 Million Wells Fargo Unsecured Credit Facility | LIBOR
Debt Instrument [Line Items]
Interest rate margin (as a percent)							1.15%
Unsecured term loans | $150 Million Wells Fargo Unsecured Term Loan C | Unsecured line of credit
Debt Instrument [Line Items]
Debt instrument, face amount	150,000,000.0				$ 150,000,000		$ 150,000,000
Line of credit facility, accordion feature, increase limit	250,000,000.0
Deferred financing fees	$ 1,000,000
Commitment fee percentage	0.50%
Unused capacity fee percentage	0.175%
Annual fee	$ 50,000
Debt issuance costs, term	5 years
Line of credit facility, accordion feature, number of advances | advance	6
Line of credit facility, accordion feature, minimum advances	$ 25,000,000.0
Unsecured term loans | $150 Million Wells Fargo Unsecured Term Loan C | Unsecured line of credit | LIBOR
Debt Instrument [Line Items]
Interest rate margin (as a percent)							1.30%
Unsecured term loans | $150 Million Wells Fargo Unsecured Term Loan C | Unsecured line of credit | LIBOR | Minimum
Debt Instrument [Line Items]
Interest rate margin (as a percent)	1.30%
Unsecured term loans | $150 Million Wells Fargo Unsecured Term Loan C | Unsecured line of credit | LIBOR | Maximum
Debt Instrument [Line Items]
Interest rate margin (as a percent)	1.90%
Unsecured term loans | $150 Million Wells Fargo Unsecured Term Loan C | Unsecured line of credit | Base rate | Minimum
Debt Instrument [Line Items]
Interest rate margin (as a percent)	0.30%
Unsecured term loans | $150 Million Wells Fargo Unsecured Term Loan C | Unsecured line of credit | Base rate | Maximum
Debt Instrument [Line Items]
Interest rate margin (as a percent)	0.90%