Summary of Significant Accounting Policies - Revenue (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Revenue from External Customer
Revenues	$ 1,904,998	$ 2,218,132	$ 2,047,579
Leasing and other commissions
Revenue from External Customer
Revenues	513,842	854,780	817,435
Capital markets commissions
Revenue from External Customer
Revenues	454,106	541,255	468,904
Gains from mortgage banking activities/originations, net
Revenue from External Customer
Revenues	310,914	198,085	182,264
Management services, servicing fees and other
Revenue from External Customer
Revenues	$ 626,136	$ 624,012	$ 578,976