Leases - Third Party Leases, Cost and Accumulated Depreciation (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Lease payments to related parties	$ 0	$ 300
Office space leased in headquarters office and to third party lessees
Accumulated Depreciation	$ 244,421	230,186
Terms of such leases
Lessor, renewal options included	true
Operating lease income related to lease payments
Operating lease income	$ 6,300	5,900
Lease income related to variable lease payments	5,600	5,300
Future minimum rental income from related parties
2021	6,454
2022	4,724
2023	3,800
2024	2,973
2025	2,404
Thereafter	5,932
Third party lessees
Office space leased in headquarters office and to third party lessees
Cost related to leased properties	304,300	288,800
Accumulated Depreciation	$ 150,000	141,300
Terms of such leases
Lessor, Operating Lease, Renewal Term	10 years
Operating lease income related to lease payments
Operating lease income	$ 400	$ 400
Future minimum rental income from related parties
2021	400
2022	400
2023	400
2024	400
2025	400
Thereafter	$ 400