Significant Accounting Policies (Narrative) (Details) $ in Thousands	9 Months Ended
	Dec. 31, 2020 USD ($) item	Sep. 30, 2020 USD ($)	Jun. 30, 2020 USD ($)	Mar. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)	Sep. 30, 2019 USD ($)	Jun. 30, 2019 USD ($)	Mar. 31, 2019 USD ($)
Significant Accounting Policies [Line Items]
Allowance for credit losses	$ 24,003			$ 22,360
Cumulative effect	3,233,738	$ 3,209,250	$ 3,239,354	3,285,447	1,950,131	$ 1,886,107	$ 1,131,424	1,132,487
ROU assets	99,258
Operating lease liability	$ 111,602
Minimum [Member]
Significant Accounting Policies [Line Items]
Number of lease renewal options | item	1
Lease renewal option, extension period	1 month
Maximum [Member]
Significant Accounting Policies [Line Items]
Lease renewal option, extension period	1 year
Accumulated Deficit [Member]
Significant Accounting Policies [Line Items]
Cumulative effect	$ (1,029,601)	$ (1,031,797)	$ (989,175)	(930,064)	$ (63,183)	$ (113,918)	$ (19,983)	(17,841)
ASC 842 [Member]
Significant Accounting Policies [Line Items]
ROU assets				111,815
Operating lease liability				125,331
Cumulative Effect, Period of Adoption, Adjustment [Member] | ASU 2016-13 [Member]
Significant Accounting Policies [Line Items]
Allowance for credit losses				417
Cumulative effect				(417)
Cumulative Effect, Period of Adoption, Adjustment [Member] | ASU 2016-13 [Member] | Accumulated Deficit [Member]
Significant Accounting Policies [Line Items]
Cumulative effect				$ (417)