Leases (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Lessee, Lease, Description [Line Items]
Operating Lease, Right-of-Use Asset	$ 15,078	$ 13,714
Operating Lease, Liability	16,053	14,482
Deferred Rent Credit			$ 600
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest	$ (1,040,256)	$ (969,014)
Operating Lease, Weighted Average Remaining Lease Term	7 years 2 months 12 days	7 years 2 months 12 days
Operating Lease, Weighted Average Discount Rate, Percent	2.30%	3.10%
Retained Earnings
Lessee, Lease, Description [Line Items]
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest	$ (704,764)	$ (646,847)	(614,069)	$ (561,908)
Accounting Standards Update 2016-02 [Member] | Retained Earnings
Lessee, Lease, Description [Line Items]
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest			143
Other Liabilities [Member]
Lessee, Lease, Description [Line Items]
Operating Lease, Liability	16,053	14,500	11,800
Other Assets [Member]
Lessee, Lease, Description [Line Items]
Operating Lease, Right-of-Use Asset	$ 15,100	$ 13,700	$ 11,000