Leases (Details) £ in Thousands, $ in Thousands	3 Months Ended
	Mar. 31, 2021 USD ($) Floor	Mar. 31, 2020 USD ($)	Mar. 31, 2021 GBP (£)	Dec. 31, 2020 USD ($)
Lessee, Lease, Description [Line Items]
Operating Lease, Cost	$ 12,166	$ 11,916
Variable Lease, Cost	1,852	1,723
Operating Lease, Payments	10,091	9,102
Operating Lease, Incentive Payments Received	3,441	1,270
Lessee, Additional Payments for Operating Leases Not Yet Commenced	$ 195,299
Minimum [Member]
Lessee, Lease, Description [Line Items]
Lessee, Operating Lease, Lease Not yet Commenced, Term of Contract	3 years		3 years
Maximum [Member]
Lessee, Lease, Description [Line Items]
Lessee, Operating Lease, Lease Not yet Commenced, Term of Contract	13 years		13 years
Office Equipment [Member]
Lessee, Lease, Description [Line Items]
Operating Lease, Cost	$ 1,507	$ 1,196
Building [Member]
Lessee, Lease, Description [Line Items]
Lessee, Additional Payments for Operating Leases Not Yet Commenced | £			£ 1,325
Operating Lease Additional Floors | Floor	4
Letter of Credit [Member]
Lessee, Lease, Description [Line Items]
Other Assets	$ 5,550			$ 5,550