Leases (Narrative) (Details) - USD ($)	Mar. 31, 2021	Dec. 31, 2020
Lessee, Lease, Description [Line Items]
Weighted average remaining lease term	14 years 7 months 6 days
Discount rate	3.90%
Weighted average remaining finance lease term	3 years 1 month 6 days
Finance discount rate	3.70%
Net book value	$ 483,981,000	$ 492,127,000
Minimum [Member]
Lessee, Lease, Description [Line Items]
Operating lease fixed term	1 month
Finance lease term	1 month
Maximum [Member]
Lessee, Lease, Description [Line Items]
Operating lease fixed term	20 years
Finance lease term	6 years
Computer Equipment [Member]
Lessee, Lease, Description [Line Items]
Net book value	$ 8,225,000	8,280,000
Yard And Others [Member]
Lessee, Lease, Description [Line Items]
Net book value	16,997,000	17,369,000
Finance leases [Member]
Lessee, Lease, Description [Line Items]
Net book value	25,222,000	25,649,000
Net book value	$ 25,222,000	$ 25,649,000