Leases (Details) £ in Thousands, ft² in Thousands, $ in Thousands	3 Months Ended			6 Months Ended
	Jun. 30, 2021 USD ($) ft²	Jun. 30, 2021 GBP (£)	Jun. 30, 2020 USD ($)	Jun. 30, 2021 USD ($) ft²	Jun. 30, 2020 USD ($)	Dec. 31, 2020 USD ($)
Lessee, Lease, Description [Line Items]
Operating Lease, Cost	$ 12,334		$ 12,108	$ 24,500	$ 24,024
Variable Lease, Cost	1,766		1,132	3,618	2,855
Operating Lease, Payments				22,893	11,763
Operating Lease, Incentive Payments Received				4,144	8,796
Lessee, Additional Payments for Operating Leases Not Yet Commenced	$ 198,704			$ 198,704
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	$ 1,144		$ 1,132	$ 2,651	$ 2,328
Building [Member] | UNITED KINGDOM
Lessee, Lease, Description [Line Items]
Net Rentable Area | ft²	14			14
Building [Member] | Annual Expense | UNITED KINGDOM
Lessee, Lease, Description [Line Items]
Operating Lease, Cost | £		£ 1,081
Letter of Credit [Member]
Lessee, Lease, Description [Line Items]
Other Assets	$ 5,616			$ 5,616		$ 5,550