Debt - Summary (Details) - USD ($) $ in Thousands		9 Months Ended	12 Months Ended
	Sep. 30, 2017	Sep. 30, 2017	Dec. 31, 2016	Jul. 28, 2017
Debt Instrument [Line Items]
Principal outstanding	$ 1,147,929	$ 1,147,929	$ 1,036,139
Unused borrowing capacity	$ 349,100	$ 349,100
Weighted average interest rate	3.44%	3.44%
Interest Rate Swaps
Debt Instrument [Line Items]
Notional amount	$ 450,000	$ 450,000
One Month LIBOR
Debt Instrument [Line Items]
Variable rate basis reference rate (as a percent)	1.23222%	1.23222%
Secured Debt
Debt Instrument [Line Items]
Principal outstanding	$ 58,695	$ 58,695	163,565
Long-term debt, gross	59,299	59,299	164,326
Unamortized debt issuance costs	(667)	(667)	(873)
Unamortized fair market value premium	63	63	112
Line of Credit | Unsecured Debt
Debt Instrument [Line Items]
Principal outstanding	245,000	245,000	28,000
Line of Credit | $450 Million Wells Fargo Unsecured Credit Facility | Unsecured Debt
Debt Instrument [Line Items]
Principal outstanding	245,000	245,000	$ 28,000
Maximum borrowing capacity	450,000	$ 450,000
Line of Credit | $450 Million Wells Fargo Unsecured Credit Facility | Unsecured Debt | LIBOR
Debt Instrument [Line Items]
Interest rate margin (as a percent)		1.15%	1.15%
Term Loan | Unsecured Debt
Debt Instrument [Line Items]
Principal outstanding	446,066	$ 446,066	$ 446,608
Long-term debt, gross	450,000	450,000	450,000
Unamortized debt issuance costs	(3,934)	(3,934)	(3,392)
$150 Million Wells Fargo Unsecured Term Loan C | Unsecured Debt
Debt Instrument [Line Items]
Long-term debt, gross	150,000	$ 150,000	$ 150,000
$150 Million Wells Fargo Unsecured Term Loan C | Unsecured Debt | LIBOR
Debt Instrument [Line Items]
Interest rate margin (as a percent)		1.30%	1.30%
$150 Million Wells Fargo Unsecured Term Loan B | Unsecured Debt
Debt Instrument [Line Items]
Long-term debt, gross	150,000	$ 150,000	$ 150,000
$150 Million Wells Fargo Unsecured Term Loan B | Unsecured Debt | LIBOR
Debt Instrument [Line Items]
Interest rate margin (as a percent)		1.30%	1.30%
$150 Million Wells Fargo Unsecured Term Loan A | Unsecured Debt
Debt Instrument [Line Items]
Long-term debt, gross	150,000	$ 150,000	$ 150,000
$150 Million Wells Fargo Unsecured Term Loan A | Unsecured Debt | LIBOR
Debt Instrument [Line Items]
Interest rate margin (as a percent)		1.30%	1.30%
$150 Million Unsecured Term Loan D [Member] | Unsecured Debt
Debt Instrument [Line Items]
Long-term debt, gross	$ 0	$ 0	$ 0
Unused borrowing capacity				$ 150,000
$150 Million Unsecured Term Loan D [Member] | Unsecured Debt | LIBOR
Debt Instrument [Line Items]
Interest rate margin (as a percent)	1.30%		0.00%
Unsecured Notes | Unsecured Debt
Debt Instrument [Line Items]
Principal outstanding	$ 398,168	398,168	$ 397,966
Long-term debt, gross	400,000	400,000	400,000
Unamortized debt issuance costs	(1,832)	(1,832)	(2,034)
$100 Million Series F Unsecured Notes | Unsecured Debt
Debt Instrument [Line Items]
Long-term debt, gross	$ 100,000	$ 100,000	100,000
Stated interest rate	3.98%	3.98%
$50 Million Series A Unsecured Notes | Unsecured Debt
Debt Instrument [Line Items]
Long-term debt, gross	$ 50,000	$ 50,000	50,000
Stated interest rate	4.98%	4.98%
$100 Million Series D Unsecured Notes | Unsecured Debt
Debt Instrument [Line Items]
Long-term debt, gross	$ 100,000	$ 100,000	100,000
Stated interest rate	4.32%	4.32%
$50 Million Series B Unsecured Notes | Unsecured Debt
Debt Instrument [Line Items]
Long-term debt, gross	$ 50,000	$ 50,000	50,000
Stated interest rate	4.98%	4.98%
$80 Million Series C Unsecured Notes | Unsecured Debt
Debt Instrument [Line Items]
Long-term debt, gross	$ 80,000	$ 80,000	80,000
Stated interest rate	4.42%	4.42%
$20 Million Series E Unsecured Notes | Unsecured Debt
Debt Instrument [Line Items]
Long-term debt, gross	$ 20,000	$ 20,000	20,000
Stated interest rate	4.42%	4.42%
Union Fidelity Life Insurance Co Due April 30, 2017 | Secured Debt
Debt Instrument [Line Items]
Long-term debt, gross	$ 0	$ 0	5,384
Stated interest rate	5.81%	5.81%
Penalty free prepayment period		2 months
Webster Bank, National Association due May 31, 2017 [Member] | Secured Debt
Debt Instrument [Line Items]
Penalty free prepayment period		3 months
Webster Bank, National Association due May 29, 2017 | Secured Debt
Debt Instrument [Line Items]
Long-term debt, gross	$ 0	$ 0	2,853
Stated interest rate	3.66%	3.66%
Webster Bank, National Association due May 31, 2017 | Secured Debt
Debt Instrument [Line Items]
Long-term debt, gross	$ 0	$ 0	3,073
Stated interest rate	3.64%	3.64%
Wells Fargo, National Association Due August 1, 2017 | Secured Debt
Debt Instrument [Line Items]
Long-term debt, gross	$ 0	$ 0	4,043
Stated interest rate	5.90%	5.90%
Penalty free prepayment period		3 months
Connecticut General Life Insurance Company - Facility 1 | Secured Debt
Debt Instrument [Line Items]
Long-term debt, gross	$ 0	$ 0	35,320
Stated interest rate	6.50%	6.50%
Connecticut General Life Insurance Company - Facility 2 | Secured Debt
Debt Instrument [Line Items]
Long-term debt, gross	$ 0	$ 0	36,892
Stated interest rate	5.75%	5.75%
Connecticut General Life Insurance Company - Facility 3 | Secured Debt
Debt Instrument [Line Items]
Long-term debt, gross	$ 0	$ 0	16,141
Stated interest rate	5.88%	5.88%
Wells Fargo Bank, National Association CMBS Loan | Secured Debt
Debt Instrument [Line Items]
Long-term debt, gross	$ 55,366	$ 55,366	56,608
Stated interest rate	4.31%	4.31%
Penalty free prepayment period		3 months
Thrivent Financial for Lutherans Due December 15, 2023 [Member] | Secured Debt
Debt Instrument [Line Items]
Long-term debt, gross	$ 3,933	$ 3,933	4,012
Stated interest rate	4.78%	4.78%
Connecticut General Life Insurance Company Facilities | Secured Debt
Debt Instrument [Line Items]
Penalty free prepayment period		6 months
Prepaid Expenses and Other Assets
Debt Instrument [Line Items]
Unamortized debt issuance costs	$ (1,700)	$ (1,700)	$ (2,300)