Notes and Bonds Payable - Narrative (Details)				3 Months Ended		12 Months Ended
	May 31, 2019 USD ($) extension_option	Jul. 05, 2016 USD ($)	Apr. 24, 2015 USD ($) swap_agreement	Dec. 31, 2019 USD ($)	Jun. 30, 2019 USD ($)	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Dec. 18, 2017 Lender	Dec. 11, 2017 USD ($)	Feb. 28, 2014 USD ($) Lender	Mar. 26, 2013 USD ($)	Oct. 14, 2011 USD ($)
Debt Instrument [Line Items]
Unsecured term loan outstanding				$ 1,422,258,000		$ 1,422,258,000
Notes and bonds payable				1,414,069,000		1,414,069,000	$ 1,345,984,000
Issuance costs				5,841,000		$ 5,841,000
Line of Credit [Member] | Unsecured Credit Facility [Member]
Debt Instrument [Line Items]
Unsecured credit facility													$ 700,000,000.0
Number of extension options | extension_option	2
Period of extension option	6 months
Extension fees as percentage of aggregate commitments	0.0625%
Facility fee on the aggregate amount of commitments						0.20%
Amount outstanding under Unsecured Credit Facility				$ 293,000,000.0		$ 293,000,000.0
Weighted average interest rate (LOC)				2.66%		2.66%
Remaining borrowing capacity				$ 407,000,000.0		$ 407,000,000.0
Notes and bonds payable				$ 293,000,000		$ 293,000,000	262,000,000
Line of Credit [Member] | Unsecured Credit Facility [Member] | Minimum [Member]
Debt Instrument [Line Items]
Facility fee on the aggregate amount of commitments						0.125%
Line of Credit [Member] | Unsecured Credit Facility [Member] | Maximum [Member]
Debt Instrument [Line Items]
Facility fee on the aggregate amount of commitments						0.30%
Line of Credit [Member] | Unsecured Credit Facility [Member] | LIBOR [Member]
Debt Instrument [Line Items]
Percentage of interest in addition to LIBOR						0.90%
Contractual interest rates, basis spread on variable rate						0.90%
Line of Credit [Member] | Unsecured Credit Facility [Member] | LIBOR [Member] | Minimum [Member]
Debt Instrument [Line Items]
Percentage of interest in addition to LIBOR						0.775%
Line of Credit [Member] | Unsecured Credit Facility [Member] | LIBOR [Member] | Maximum [Member]
Debt Instrument [Line Items]
Percentage of interest in addition to LIBOR						1.45%
Line of Credit [Member] | Term Loan due 2024 [Member] | LIBOR [Member]
Debt Instrument [Line Items]
Contractual interest rates, basis spread on variable rate						1.00%
Medium-term Notes [Member] | Term Loan due 2024 [Member]
Debt Instrument [Line Items]
Weighted average interest rate (LOC)				2.29%		2.29%
Senior notes face value	$ 200,000,000			$ 200,000,000		$ 200,000,000	150,000,000				$ 200,000,000.0
Number of lenders, term loan | Lender									9		9
Amount of debt repaid		$ 50,000,000.0
Contractual interest rates, basis spread on variable rate						1.00%
Unsecured term loan outstanding				$ 200,000,000.0		$ 200,000,000.0
Interest rate yielded per annum upon issuance				3.27%		3.27%
Amortization of debt issuance costs					$ 300,000	$ 200,000	200,000	$ 200,000
Notes and bonds payable				$ 199,013,000		199,013,000	149,183,000
Deferred finance costs, gross	700,000
Issuance costs	400,000			987,000		987,000	817,000
Medium-term Notes [Member] | Term Loan due 2024 [Member] | Interest Rate Swap [Member]
Debt Instrument [Line Items]
Derivative notional amount				$ 175,000,000.0		$ 175,000,000.0
Medium-term Notes [Member] | Term Loan due 2024 [Member] | Minimum [Member]
Debt Instrument [Line Items]
Contractual interest rates, basis spread on variable rate						0.85%
Medium-term Notes [Member] | Term Loan due 2024 [Member] | Maximum [Member]
Debt Instrument [Line Items]
Contractual interest rates, basis spread on variable rate						1.65%
Medium-term Notes [Member] | Term Loan due 2022 [Member]
Debt Instrument [Line Items]
Senior notes face value	$ 150,000,000.0
Medium-term Notes [Member] | Term Loan due 2026 [Member]
Debt Instrument [Line Items]
Contractual interest rates, basis spread on variable rate				1.60%
Amortization of debt issuance costs					$ 400,000
Delayed draw feature period	9 months
Anticipated amount of investment	$ 150,000,000.0
Notes and bonds payable				$ 0		$ 0
Ticking fee				0.20%
Deferred finance costs, gross	1,100,000
Issuance costs	$ 700,000
Medium-term Notes [Member] | Term Loan due 2026 [Member] | Minimum [Member]
Debt Instrument [Line Items]
Contractual interest rates, basis spread on variable rate	1.45%
Ticking fee	0.125%
Medium-term Notes [Member] | Term Loan due 2026 [Member] | Maximum [Member]
Debt Instrument [Line Items]
Contractual interest rates, basis spread on variable rate	2.40%
Ticking fee	0.30%
Medium-term Notes [Member] | Term Loan due 2026 [Member] | LIBOR [Member]
Debt Instrument [Line Items]
Contractual interest rates, basis spread on variable rate						1.60%
Senior Notes [Member] | Term Loan due 2026 [Member]
Debt Instrument [Line Items]
Senior notes face value				$ 150,000,000		$ 150,000,000
Notes and bonds payable				0		0	0
Senior Notes [Member] | Senior Notes due 2023 [Member]
Debt Instrument [Line Items]
Senior notes face value				250,000,000		250,000,000	250,000,000					$ 250,000,000.0
Interest rate yielded per annum upon issuance												3.95%
Amortization of debt issuance costs						200,000	200,000	200,000
Notes and bonds payable				248,540,000		248,540,000	248,117,000
Deferred finance costs, gross												$ 2,100,000
Issuance costs				$ 699,000		$ 699,000	909,000
Interest rate on senior notes				3.75%		3.75%						3.75%
Unaccreted discount				$ 761,000		$ 761,000	974,000					$ 2,100,000
Amount of amortization included in interest expense on the Company's consolidated Statements of operations						200,000	200,000	200,000
Senior Notes [Member] | Senior Notes due 2025 [Member]
Debt Instrument [Line Items]
Senior notes face value			$ 250,000,000.0	250,000,000		250,000,000	250,000,000
Interest rate yielded per annum upon issuance			4.08%
Amortization of debt issuance costs						200,000	200,000	200,000
Notes and bonds payable				248,522,000		248,522,000	248,278,000
Deferred finance costs, gross			$ 2,300,000
Issuance costs				$ 1,357,000		$ 1,357,000	1,581,000
Interest rate on senior notes			3.875%	3.88%		3.88%
Unaccreted discount			$ 200,000	$ 121,000		$ 121,000	141,000
Number of forward starting swaps | swap_agreement			4
Senior Notes [Member] | Senior Notes due 2025 [Member] | Interest Rate Swap [Member]
Debt Instrument [Line Items]
Settlement of swap agreements			$ 1,700,000
Senior Notes [Member] | Senior Notes due 2028 [Member]
Debt Instrument [Line Items]
Senior notes face value				300,000,000		300,000,000	300,000,000			$ 300,000,000.0
Interest rate yielded per annum upon issuance										3.84%
Amortization of debt issuance costs						200,000
Notes and bonds payable				295,651,000		295,651,000	295,198,000
Deferred finance costs, gross										$ 2,700,000
Issuance costs				$ 2,249,000		$ 2,249,000	2,483,000
Interest rate on senior notes				3.63%		3.63%				3.625%
Unaccreted discount				$ 2,100,000		$ 2,100,000	2,319,000			$ 2,500,000
Amount of amortization included in interest expense on the Company's consolidated Statements of operations						200,000
Mortgage Notes [Member]
Debt Instrument [Line Items]
Issuance costs				$ 500,000		$ 500,000
Mortgage Notes [Member] | Minimum [Member]
Debt Instrument [Line Items]
Interest rate on senior notes				3.30%		3.30%
Mortgage Notes [Member] | Maximum [Member]
Debt Instrument [Line Items]
Interest rate on senior notes				6.90%		6.90%
Mortgage Notes [Member] | Mortgage Notes Payable [Member]
Debt Instrument [Line Items]
Senior notes face value				$ 129,258,000		$ 129,258,000	143,115,000
Amortization of debt issuance costs						200,000	100,000
Notes and bonds payable				129,343,000		129,343,000	143,208,000
Issuance costs				549,000		549,000	744,000
Unaccreted discount				$ 528,000		528,000	968,000
Amortization of debt discount						600,000	400,000	300,000
Amortization of debt premium						$ 400,000	$ 800,000	$ 700,000
Mortgage Notes [Member] | Mortgage Notes Payable [Member] | Minimum [Member]
Debt Instrument [Line Items]
Interest rate on senior notes				3.31%		3.31%
Mortgage Notes [Member] | Mortgage Notes Payable [Member] | Maximum [Member]
Debt Instrument [Line Items]
Interest rate on senior notes				6.88%		6.88%
Up-front Payment Arrangement [Member] | Line of Credit [Member] | Unsecured Credit Facility [Member]
Debt Instrument [Line Items]
Fee amount				$ 3,500,000		$ 3,500,000