Notes and Bonds Payable (Details) - USD ($)		3 Months Ended	12 Months Ended
	May 31, 2019	Dec. 31, 2019	Dec. 31, 2019	Dec. 31, 2018	Dec. 11, 2017	Apr. 24, 2015	Feb. 28, 2014	Mar. 26, 2013
Debt Instrument [Line Items]
Notes and bonds payable		$ 1,414,069,000	$ 1,414,069,000	$ 1,345,984,000
Line of Credit [Member] | Unsecured Credit Facility [Member]
Debt Instrument [Line Items]
Notes and bonds payable		293,000,000	293,000,000	262,000,000
Medium-term Notes [Member] | Term Loan due 2022 [Member]
Debt Instrument [Line Items]
Senior and term notes face value	$ 150,000,000.0
Medium-term Notes [Member] | Term Loan due 2024 [Member]
Debt Instrument [Line Items]
Notes and bonds payable		199,013,000	$ 199,013,000	149,183,000
Contractual interest rates, basis spread on variable rate			1.00%
Senior and term notes face value	$ 200,000,000	200,000,000	$ 200,000,000	150,000,000			$ 200,000,000.0
Medium-term Notes [Member] | Term Loan due 2026 [Member]
Debt Instrument [Line Items]
Notes and bonds payable		$ 0	0
Contractual interest rates, basis spread on variable rate		1.60%
Senior Notes [Member] | Term Loan due 2026 [Member]
Debt Instrument [Line Items]
Notes and bonds payable		$ 0	0	0
Senior and term notes face value		150,000,000	150,000,000
Senior Notes [Member] | Senior Notes due 2023 [Member]
Debt Instrument [Line Items]
Notes and bonds payable		$ 248,540,000	$ 248,540,000	248,117,000
Contractual interest rates		3.75%	3.75%					3.75%
Senior and term notes face value		$ 250,000,000	$ 250,000,000	250,000,000				$ 250,000,000.0
Senior Notes [Member] | Senior Notes due 2025 [Member]
Debt Instrument [Line Items]
Notes and bonds payable		$ 248,522,000	$ 248,522,000	248,278,000
Contractual interest rates		3.88%	3.88%			3.875%
Senior and term notes face value		$ 250,000,000	$ 250,000,000	250,000,000		$ 250,000,000.0
Senior Notes [Member] | Senior Notes due 2028 [Member]
Debt Instrument [Line Items]
Notes and bonds payable		$ 295,651,000	$ 295,651,000	295,198,000
Contractual interest rates		3.63%	3.63%		3.625%
Senior and term notes face value		$ 300,000,000	$ 300,000,000	300,000,000	$ 300,000,000.0
Mortgage Notes [Member] | Mortgage Notes Payable [Member]
Debt Instrument [Line Items]
Notes and bonds payable		129,343,000	129,343,000	143,208,000
Senior and term notes face value		$ 129,258,000	$ 129,258,000	$ 143,115,000
LIBOR [Member] | Line of Credit [Member] | Unsecured Credit Facility [Member]
Debt Instrument [Line Items]
Contractual interest rates, basis spread on variable rate			0.90%
LIBOR [Member] | Line of Credit [Member] | Term Loan due 2024 [Member]
Debt Instrument [Line Items]
Contractual interest rates, basis spread on variable rate			1.00%
LIBOR [Member] | Medium-term Notes [Member] | Term Loan due 2026 [Member]
Debt Instrument [Line Items]
Contractual interest rates, basis spread on variable rate			1.60%
Minimum [Member] | Medium-term Notes [Member] | Term Loan due 2024 [Member]
Debt Instrument [Line Items]
Contractual interest rates, basis spread on variable rate			0.85%
Minimum [Member] | Medium-term Notes [Member] | Term Loan due 2026 [Member]
Debt Instrument [Line Items]
Contractual interest rates, basis spread on variable rate	1.45%
Minimum [Member] | Mortgage Notes [Member]
Debt Instrument [Line Items]
Contractual interest rates		3.30%	3.30%
Minimum [Member] | Mortgage Notes [Member] | Mortgage Notes Payable [Member]
Debt Instrument [Line Items]
Contractual interest rates		3.31%	3.31%
Maximum [Member] | Medium-term Notes [Member] | Term Loan due 2024 [Member]
Debt Instrument [Line Items]
Contractual interest rates, basis spread on variable rate			1.65%
Maximum [Member] | Medium-term Notes [Member] | Term Loan due 2026 [Member]
Debt Instrument [Line Items]
Contractual interest rates, basis spread on variable rate	2.40%
Maximum [Member] | Mortgage Notes [Member]
Debt Instrument [Line Items]
Contractual interest rates		6.90%	6.90%
Maximum [Member] | Mortgage Notes [Member] | Mortgage Notes Payable [Member]
Debt Instrument [Line Items]
Contractual interest rates		6.88%	6.88%