Corporate Indebtedness - Senior Unsecured Notes (Details) - USD ($)	1 Months Ended	9 Months Ended
	Oct. 31, 2017	Sep. 30, 2021	Dec. 31, 2020
Debt disclosures by scheduled maturity date
Accreted Balance		$ 1,496,583,000	$ 1,495,275,000
Senior unsecured notes
Debt disclosures by scheduled maturity date
Unpaid Principal Balance		900,000,000
Accreted Balance		901,160,000	1,251,889,000
Deferred financing costs, net		(2,042,000)	(3,670,000)
Accreted Balance, net of deferred financing costs		899,118,000	1,248,219,000
Senior unsecured notes | 3.25 Percent Senior Unsecured Notes Due October 2022
Debt disclosures by scheduled maturity date
Unpaid Principal Balance		500,000,000
Accreted Balance		$ 499,885,000	499,803,000
Coupon Rate (as a percent)		3.25%
Initial Term (in Years)		5 years
Senior unsecured notes | 4.50% Senior unsecured notes maturing on December 1, 2022
Debt disclosures by scheduled maturity date
Unpaid Principal Balance		$ 300,000,000
Accreted Balance		$ 301,275,000	302,086,000
Coupon Rate (as a percent)	4.50%	4.50%
Initial Term (in Years)		10 years
Face amount of loan	$ 100,000,000
Redemption price, percentage	105.334%
Senior unsecured notes | 4.27% Senior unsecured notes maturing on December 17, 2025
Debt disclosures by scheduled maturity date
Unpaid Principal Balance		$ 100,000,000
Accreted Balance		$ 100,000,000	100,000,000
Coupon Rate (as a percent)		4.27%
Initial Term (in Years)		10 years
Senior unsecured notes | Senior Unsecured Notes Due August 2021
Debt disclosures by scheduled maturity date
Unpaid Principal Balance		$ 0
Accreted Balance		$ 0	$ 350,000,000
Coupon Rate (as a percent)		0.00%
Initial Term (in Years)		3 years