Fair Value Measurements (Financial Assets and Liabilities Measured at Fair Value on a Recurring Basis) (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Financial Assets and Liabilities Measured at Fair Value on a Nonrecurring Basis
Marketable securities	$ 0	$ 33,313	$ 152,198
Real estate fund investments	3,739	222,649
Real estate fund investments
Financial Assets and Liabilities Measured at Fair Value on a Nonrecurring Basis
Real estate fund investments	3,739	222,649
Recurring
Financial Assets and Liabilities Measured at Fair Value on a Nonrecurring Basis
Marketable securities		33,313
Real estate fund investments	3,739	222,649
Deferred compensation plan assets (included in restricted cash and other assets)	105,564	103,773
Loans receivable (included in investments in partially owned entities and in other assets)	47,743
Interest rate caps (included in other assets)	17	4,327
Total assets	157,063	364,062
Mandatorily redeemable instruments (included in other liabilities)	50,002	50,561
Interest rate swaps (included in other liabilities)	66,033	40,354
Total liabilities	116,035	90,915
Recurring | Level 1
Financial Assets and Liabilities Measured at Fair Value on a Nonrecurring Basis
Marketable securities		33,313
Real estate fund investments	0	0
Deferred compensation plan assets (included in restricted cash and other assets)	65,636	71,338
Loans receivable (included in investments in partially owned entities and in other assets)	0
Interest rate caps (included in other assets)	0	0
Total assets	65,636	104,651
Mandatorily redeemable instruments (included in other liabilities)	50,002	50,561
Interest rate swaps (included in other liabilities)	0	0
Total liabilities	50,002	50,561
Recurring | Level 2
Financial Assets and Liabilities Measured at Fair Value on a Nonrecurring Basis
Marketable securities		0
Real estate fund investments	0	0
Deferred compensation plan assets (included in restricted cash and other assets)	0	0
Loans receivable (included in investments in partially owned entities and in other assets)	0
Interest rate caps (included in other assets)	17	4,327
Total assets	17	4,327
Mandatorily redeemable instruments (included in other liabilities)	0	0
Interest rate swaps (included in other liabilities)	66,033	40,354
Total liabilities	66,033	40,354
Recurring | Level 3
Financial Assets and Liabilities Measured at Fair Value on a Nonrecurring Basis
Marketable securities		0
Real estate fund investments	3,739	222,649
Deferred compensation plan assets (included in restricted cash and other assets)	39,928	32,435
Loans receivable (included in investments in partially owned entities and in other assets)	47,743
Interest rate caps (included in other assets)	0	0
Total assets	91,410	255,084
Mandatorily redeemable instruments (included in other liabilities)	0	0
Interest rate swaps (included in other liabilities)	0	0
Total liabilities	0	0
Recurring | Level 3 | Real estate fund investments
Financial Assets and Liabilities Measured at Fair Value on a Nonrecurring Basis
Real estate fund investments	3,739
Restricted Cash | Recurring
Financial Assets and Liabilities Measured at Fair Value on a Nonrecurring Basis
Deferred compensation plan assets (included in restricted cash and other assets)	10,813	11,819
Other Assets | Recurring
Financial Assets and Liabilities Measured at Fair Value on a Nonrecurring Basis
Deferred compensation plan assets (included in restricted cash and other assets)	94,751	$ 91,954
Loans receivable (included in investments in partially owned entities and in other assets)	4,735
Partially Owned Properties | Recurring
Financial Assets and Liabilities Measured at Fair Value on a Nonrecurring Basis
Loans receivable (included in investments in partially owned entities and in other assets)	$ 43,008