ACCUMULATED OTHER COMPREHENSIVE INCOME (Summary of Changes in Accumulated Other Comprehensive Income) (Details) - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance at the beginning of the period		$ 3,332,988	$ 3,238,126	$ 3,188,551
Other comprehensive income (loss) before reclassifications		(32,134)	(19,318)	2,120
(Gain) loss reclassified from accumulated other comprehensive loss to net income		9,064	(1,939)	(2,153)
Adoption of ASU 2018-02	[1]	0	0	(1,148)
Pension adjustment	[2]	(84)	11	759
Share of investee's other comprehensive income, net of tax of $285	[3]	1,002	0	0
Deconsolidation of 110 North Wacker		12,934
Net current-period other comprehensive income (loss)		(9,218)	(21,246)	(1,161)
Balance at the end of the period		3,715,023	3,332,988	3,238,126
Share of investee's other comprehensive income, tax expense		285
Cumulative Effect, Period of Adoption, Adjustment
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance at the beginning of the period	[4]	(18)
Balance at the end of the period	[4]		(18)
AOCI Attributable to Parent
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance at the beginning of the period		(29,372)	(8,126)	(6,965)
Adoption of ASU 2018-02				(1,148)
Pension adjustment		(84)	11	759
Share of investee's other comprehensive income, net of tax of $285		1,002
Balance at the end of the period		$ (38,590)	$ (29,372)	(8,126)
AOCI Attributable to Parent | Cumulative Effect, Period of Adoption, Adjustment
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance at the beginning of the period				$ (739)

[1] The Company adopted Accounting Standards Update (ASU) 2018-02, Income Statement-Reporting Comprehensive Income (Topic 220): Reclassification of Certain Tax Effects from Accumulated Other Comprehensive Income, as of January 1, 2018.

[2] The deferred tax impact is not meaningful for the years ended December 31, 2020 and 2019. Amount is net of deferred tax expense of $0.5 million for the year ended December 31, 2018.

[3] The amount for 2020 is shown net of deferred tax expense of $0.3 million.

[4] Related to the adoption of ASU 2016-13, Financial Instruments-Credit Losses (Topic 326) and all its related amendments as of January 1, 2020.