Notes and Bonds Payable - Senior and Term Notes (Details) - USD ($)	Dec. 31, 2018	Dec. 31, 2017	Dec. 11, 2017	Apr. 24, 2015	Feb. 28, 2014	Mar. 26, 2013
Debt Instrument [Line Items]
Issuance costs	$ (6,534,000)
Notes and bonds payable	$ 1,345,984,000	$ 1,283,880,000
Medium-term Notes [Member] | Term Loan due 2022 [Member]
Debt Instrument [Line Items]
Effective Interest rate	3.53%
Senior and term notes face value	$ 150,000,000	150,000,000			$ 200,000,000.0
Issuance costs	(817,000)	(1,006,000)
Notes and bonds payable	149,183,000	148,994,000
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	(974,000)	(1,178,000)				$ (2,100,000)
Issuance costs	(909,000)	(1,119,000)
Notes and bonds payable	248,117,000	247,703,000
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	(141,000)	(160,000)		$ (200,000)
Issuance costs	(1,581,000)	(1,796,000)
Notes and bonds payable	248,278,000	248,044,000
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	(2,319,000)	(2,529,000)	$ (2,500,000)
Issuance costs	(2,483,000)	(2,714,000)
Notes and bonds payable	$ 295,198,000	$ 294,757,000