Debt - Summary (Details) - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2017	Dec. 31, 2016
Debt Instrument [Line Items]
Principal outstanding	$ 1,121,696	$ 1,036,139
Weighted average interest rate	3.62%
Interest Rate Swaps
Debt Instrument [Line Items]
Notional amount	$ 450,000
One Month LIBOR
Debt Instrument [Line Items]
Variable rate basis reference rate (as a percent)	1.22389%
Secured Debt
Debt Instrument [Line Items]
Principal outstanding	$ 146,641	163,565
Long-term debt, gross	147,293	164,326
Unamortized debt issuance costs	(718)	(873)
Unamortized fair market value premium	66	112
Term Loan | Unsecured Debt
Debt Instrument [Line Items]
Principal outstanding	446,954	446,608
Long-term debt, gross	450,000	450,000
Unamortized debt issuance costs	(3,046)	(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 Million Wells Fargo Unsecured Term Loan C | Unsecured Debt | LIBOR
Debt Instrument [Line Items]
Interest rate margin (as a percent)	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 Million Wells Fargo Unsecured Term Loan B | Unsecured Debt | LIBOR
Debt Instrument [Line Items]
Interest rate margin (as a percent)	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 Million Wells Fargo Unsecured Term Loan A | Unsecured Debt | LIBOR
Debt Instrument [Line Items]
Interest rate margin (as a percent)	1.30%
Unsecured Notes | Unsecured Debt
Debt Instrument [Line Items]
Principal outstanding	$ 398,101	397,966
Long-term debt, gross	400,000	400,000
Unamortized debt issuance costs	(1,899)	(2,034)
$100 Million Series F Unsecured Notes | Unsecured Debt
Debt Instrument [Line Items]
Long-term debt, gross	$ 100,000	100,000
Stated interest rate	3.98%
$50 Million Series A Unsecured Notes | Unsecured Debt
Debt Instrument [Line Items]
Long-term debt, gross	$ 50,000	50,000
Stated interest rate	4.98%
$100 Million Series D Unsecured Notes | Unsecured Debt
Debt Instrument [Line Items]
Long-term debt, gross	$ 100,000	100,000
Stated interest rate	4.32%
$50 Million Series B Unsecured Notes | Unsecured Debt
Debt Instrument [Line Items]
Long-term debt, gross	$ 50,000	50,000
Stated interest rate	4.98%
$80 Million Series C Unsecured Notes | Unsecured Debt
Debt Instrument [Line Items]
Long-term debt, gross	$ 80,000	80,000
Stated interest rate	4.42%
$20 Million Series E Unsecured Notes | Unsecured Debt
Debt Instrument [Line Items]
Long-term debt, gross	$ 20,000	20,000
Stated interest rate	4.42%
Union Fidelity Life Insurance Co Due April 30, 2017 | Secured Debt
Debt Instrument [Line Items]
Long-term debt, gross	$ 0	5,384
Stated interest rate	5.81%
Penalty free prepayment period	2 months
Webster Bank, National Association due May 29, 2017 | Secured Debt
Debt Instrument [Line Items]
Long-term debt, gross	$ 0	2,853
Stated interest rate	3.66%
Webster Bank, National Association due May 31, 2017 | Secured Debt
Debt Instrument [Line Items]
Long-term debt, gross	$ 0	3,073
Stated interest rate	3.64%
Wells Fargo, National Association Due August 1, 2017 | Secured Debt
Debt Instrument [Line Items]
Long-term debt, gross	$ 0	4,043
Stated interest rate	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	$ 35,012	35,320
Stated interest rate	6.50%
Connecticut General Life Insurance Company - Facility 2 | Secured Debt
Debt Instrument [Line Items]
Long-term debt, gross	$ 36,539	36,892
Stated interest rate	5.75%
Connecticut General Life Insurance Company - Facility 3 | Secured Debt
Debt Instrument [Line Items]
Long-term debt, gross	$ 16,004	16,141
Stated interest rate	5.88%
Wells Fargo Bank, National Association CMBS Loan | Secured Debt
Debt Instrument [Line Items]
Long-term debt, gross	$ 55,778	56,608
Stated interest rate	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	$ 3,960	4,012
Stated interest rate	4.78%
Connecticut General Life Insurance Company Facilities | Secured Debt
Debt Instrument [Line Items]
Penalty free prepayment period	6 months
Line of Credit | Unsecured Debt
Debt Instrument [Line Items]
Principal outstanding	$ 130,000	28,000
Line of Credit | $450 Million Wells Fargo Unsecured Credit Facility | Unsecured Debt
Debt Instrument [Line Items]
Principal outstanding	130,000	$ 28,000
Maximum borrowing capacity	$ 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%