Notes and Bonds Payable - Senior and Term Notes (Details) - USD ($)	Dec. 31, 2019	May 31, 2019	Dec. 31, 2018	Dec. 11, 2017	Apr. 24, 2015	Feb. 28, 2014	Mar. 26, 2013
Debt Instrument [Line Items]
Debt issuance costs	$ (5,841,000)
Notes and bonds payable	$ 1,414,069,000		$ 1,345,984,000
Medium-term Notes [Member] | Term Loan due 2024 [Member]
Debt Instrument [Line Items]
Effective Interest rate	3.27%
Senior and term notes face value	$ 200,000,000	$ 200,000,000	150,000,000			$ 200,000,000.0
Debt issuance costs	(987,000)	(400,000)	(817,000)
Notes and bonds payable	199,013,000		149,183,000
Medium-term Notes [Member] | Term Loan due 2022 [Member]
Debt Instrument [Line Items]
Senior and term notes face value		$ 150,000,000.0
Senior Notes [Member] | Senior Notes due 2023 [Member]
Debt Instrument [Line Items]
Effective Interest rate							3.95%
Senior and term notes face value	250,000,000		250,000,000				$ 250,000,000.0
Unaccreted discount	(761,000)		(974,000)				$ (2,100,000)
Debt issuance costs	(699,000)		(909,000)
Notes and bonds payable	248,540,000		248,117,000
Senior Notes [Member] | Senior Notes due 2025 [Member]
Debt Instrument [Line Items]
Effective Interest rate					4.08%
Senior and term notes face value	250,000,000		250,000,000		$ 250,000,000.0
Unaccreted discount	(121,000)		(141,000)		$ (200,000)
Debt issuance costs	(1,357,000)		(1,581,000)
Notes and bonds payable	248,522,000		248,278,000
Senior Notes [Member] | Senior Notes due 2028 [Member]
Debt Instrument [Line Items]
Effective Interest rate				3.84%
Senior and term notes face value	300,000,000		300,000,000	$ 300,000,000.0
Unaccreted discount	(2,100,000)		(2,319,000)	$ (2,500,000)
Debt issuance costs	(2,249,000)		(2,483,000)
Notes and bonds payable	$ 295,651,000		$ 295,198,000