Fair Value Measurements (Financial Assets and Liabilities Measured at Fair Value on a Recurring Basis) (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Financial Assets and Liabilities Measured at Fair Value on a Recurring Basis
Marketable securities	$ 165,650	$ 182,752
Real estate fund investments	373,039	354,804
Recurring
Financial Assets and Liabilities Measured at Fair Value on a Recurring Basis
Marketable securities	165,650	182,752
Real estate fund investments	373,039	354,804
Deferred compensation plan assets (included in restricted cash and other assets)	100,368	109,177
Interest rate swaps (included in other assets)	39,584	27,472
Total assets	678,641	674,205
Mandatorily redeemable instruments (included in other liabilities)	50,561	520,561
Interest rate swap (included in other liabilities)		1,052
Total liabilities		521,613
Recurring | Level 1
Financial Assets and Liabilities Measured at Fair Value on a Recurring Basis
Marketable securities		182,752
Real estate fund investments	0	0
Deferred compensation plan assets (included in restricted cash and other assets)	60,498	69,049
Interest rate swaps (included in other assets)	0	0
Total assets	226,148	251,801
Mandatorily redeemable instruments (included in other liabilities)	50,561	520,561
Interest rate swap (included in other liabilities)		0
Total liabilities		520,561
Recurring | Level 2
Financial Assets and Liabilities Measured at Fair Value on a Recurring Basis
Marketable securities	0	0
Real estate fund investments	0	0
Deferred compensation plan assets (included in restricted cash and other assets)	0	0
Interest rate swaps (included in other assets)	39,584	27,472
Total assets	39,584	27,472
Mandatorily redeemable instruments (included in other liabilities)	0	0
Interest rate swap (included in other liabilities)		1,052
Total liabilities		1,052
Recurring | Level 3
Financial Assets and Liabilities Measured at Fair Value on a Recurring Basis
Marketable securities	0	0
Real estate fund investments		354,804
Deferred compensation plan assets (included in restricted cash and other assets)	39,870	40,128
Interest rate swaps (included in other assets)	0	0
Total assets	412,909	394,932
Mandatorily redeemable instruments (included in other liabilities)	0	0
Interest rate swap (included in other liabilities)		0
Total liabilities		0
Other Assets | Recurring
Financial Assets and Liabilities Measured at Fair Value on a Recurring Basis
Deferred compensation plan assets (included in restricted cash and other assets)	95,993	97,632
Restricted Cash | Recurring
Financial Assets and Liabilities Measured at Fair Value on a Recurring Basis
Deferred compensation plan assets (included in restricted cash and other assets)	$ 4,375	$ 11,545