Corporate Indebtedness - Senior Unsecured Notes (Details) - USD ($)	1 Months Ended	3 Months Ended
	Oct. 31, 2017	Mar. 31, 2021	Dec. 31, 2020	Apr. 30, 2020
Debt disclosures by scheduled maturity date
Accreted Balance		$ 1,495,620,000	$ 1,495,275,000
Senior unsecured notes
Debt disclosures by scheduled maturity date
Unpaid Principal Balance		1,250,000,000
Accreted Balance		1,251,647,000	1,251,889,000
Deferred financing costs, net		(3,025,000)	(3,670,000)
Accreted Balance, net of deferred financing costs		1,248,622,000	1,248,219,000
Senior unsecured notes | Senior Unsecured Notes Due August 2021
Debt disclosures by scheduled maturity date
Unpaid Principal Balance		350,000,000
Accreted Balance		$ 350,000,000	350,000,000
Coupon Rate (as a percent)		1.52%
Initial Term (in Years)		3 years
Redemption price, percentage		100.00%
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		$ 499,830,000	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		$ 301,817,000	302,086,000
Coupon Rate (as a percent)	4.50%	4.50%
Initial Term (in Years)		10 years
Face amount of loan	$ 100,000,000.0
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
Interest Rate Swap | Senior Unsecured Notes Due August 2021
Debt disclosures by scheduled maturity date
Notional amount				$ 350,000,000.0
Derivative, fixed interest rate				0.54375%