Subsequent Events (Details) $ in Millions	Nov. 09, 2018 USD ($)	Nov. 09, 2018 AUD ($)	Nov. 06, 2018 USD ($)	Jan. 23, 2018 USD ($)	Nov. 07, 2018 USD ($)	Sep. 30, 2018 USD ($)	Feb. 06, 2018 USD ($)	Dec. 31, 2017 USD ($)
Subsequent Event [Line Items]
Debt						$ 1,376,998,000
Line of Credit | 2018 Senior Secured Revolving Credit Facility secured by stock pledge in qualified subsidiaries | Revolving Credit Facility
Subsequent Event [Line Items]
Line of credit facility, maximum borrowing capacity				$ 400,000,000			$ 450,000,000
Term of debt				3 years
Debt						$ 0		$ 0
Line of Credit | 2018 Senior Secured Revolving Credit Facility secured by stock pledge in qualified subsidiaries | Revolving Credit Facility | Subsequent Event
Subsequent Event [Line Items]
Line of credit facility, maximum borrowing capacity			$ 450,000,000		$ 800,000,000
Senior Notes | 4.86% Senior Unsecured Notes | Subsequent Event
Subsequent Event [Line Items]
Notes			$ 400,000,000
Senior Notes | 4.86% Senior Unsecured Notes, 10 Year Term | Subsequent Event
Subsequent Event [Line Items]
Contractual Interest Rate			4.86%
Term of debt			10 years
Senior Notes | 4.86% Senior Unsecured Notes, 7 Year Term | Subsequent Event
Subsequent Event [Line Items]
Notes			$ 200,000,000
Contractual Interest Rate			4.68%
Term of debt			7 years
Mortgage Loans | 2010 CMBS Financing | Subsequent Event
Subsequent Event [Line Items]
Notes			$ 600,000,000
Debt			448,000,000
Write off of deferred financing costs			3,400,000
Minimum | Mortgage Loans | 2010 CMBS Financing | Subsequent Event
Subsequent Event [Line Items]
Defeasance costs			17,000,000
Maximum | Mortgage Loans | 2010 CMBS Financing | Subsequent Event
Subsequent Event [Line Items]
Defeasance costs			$ 19,000,000
Affiliate | Land | Subsequent Event
Subsequent Event [Line Items]
Purchase of land	$ 46,600,000	$ 64.5