Notes and Bonds Payable - Senior Notes (Details) - USD ($)	Dec. 31, 2016	Dec. 31, 2015	Apr. 24, 2015	Mar. 26, 2013	Dec. 13, 2010
Debt Instrument [Line Items]
Issuance costs	$ (5,620,000)
Notes and bonds payable	1,264,370,000	$ 1,424,992,000
Senior Notes [Member] | 5.750% Senior Notes Due 2021 [Member]
Debt Instrument [Line Items]
Senior notes face value	400,000,000	400,000,000			$ 400,000,000.0
Unaccreted discount	(1,510,000)	(1,832,000)
Issuance costs	(1,343,000)	(1,679,000)
Notes and bonds payable	397,147,000	396,489,000
Senior Notes [Member] | 3.75% Senior Notes Due 2023 [Member]
Debt Instrument [Line Items]
Senior notes face value	250,000,000	250,000,000		$ 250,000,000.0
Unaccreted discount	(1,375,000)	(1,565,000)		$ (2,100,000)
Issuance costs	(1,329,000)	(1,538,000)
Notes and bonds payable	247,296,000	246,897,000
Senior Notes [Member] | 3.875% Unsecured Senior Notes due 2025 [Member]
Debt Instrument [Line Items]
Senior notes face value	250,000,000	250,000,000	$ 250,000,000.0
Unaccreted discount	(178,000)	(196,000)	$ (200,000)
Issuance costs	(2,003,000)	(2,202,000)
Notes and bonds payable	$ 247,819,000	$ 247,602,000