Leases (Narrative) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Jan. 01, 2019
Lessee, Lease, Description [Line Items]
Accounting Standards Update [Extensible List]	us-gaap:AccountingStandardsUpdate201817Member	us-gaap:AccountingStandardsUpdate201802Member
Operating lease right-of-use assets	$ 145,356	$ 149,578
Operating lease liabilities	181,406	175,257
Increase to accumulated deficit	$ 653,945	678,464
Lease renewal term	5 years
Lease termination term	1 year
Amortization and impairment	$ 20,497	21,088
Interest accretion	17,012	17,489
Lease not yet commenced	2,320
Rental expense	37,509	$ 38,577
Rental expense			$ 38,893
One Lease, Lessor Affiliate of Significant Investor
Lessee, Lease, Description [Line Items]
Operating lease right-of-use assets	948
Operating lease liabilities	1,009
Rental expense	$ 458
Minimum
Lessee, Lease, Description [Line Items]
Remaining lease terms	1 year
Operating lease terms	2 years
Maximum
Lessee, Lease, Description [Line Items]
Remaining lease terms	12 years
Operating lease terms	3 years
ASU 2016-02
Lessee, Lease, Description [Line Items]
Operating lease right-of-use assets				$ 128,890
Operating lease liabilities				153,676
Deferred rent				22,881
Tenant improvements receivable				355
Increase to accumulated deficit				$ 1,550