Accounting Policies - Allowance for Doubtful Accounts Rollforward (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance - January 1	$ 8,700
Additions: Charges to costs and expenses	6,438	$ 2,928	$ 1,577
Deductions: Write-offs	(1,138)
Ending balance - December 31	$ 14,000	$ 8,700