Commitments and Contingencies (Employee Benefit Plan, Letters of Credit, and Non Income Tax Matters Narrative) (Details) - USD ($)	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019	Dec. 31, 2019
Other Commitments [Line Items]
Employer matching contributions	$ 800,000	$ 800,000	$ 1,700,000	$ 1,800,000
Estimated sales tax liability, including interest, minimum	2,500,000		2,500,000
Estimated sales tax liability, including interest, maximum	6,300,000		6,300,000
Accrued sales tax, including interest	2,500,000		2,500,000
Match Step One
Other Commitments [Line Items]
Employer matching contribution, percent of match					100.00%
Employer matching contribution percent of eligible compensation					3.00%
Match Step Two
Other Commitments [Line Items]
Employer matching contribution, percent of match					50.00%
Employer matching contribution percent of eligible compensation					2.00%
Letter of Credit
Other Commitments [Line Items]
Proceeds from amounts drawn against letters of credit			0
Letter of Credit | LOC for Office Space
Other Commitments [Line Items]
Letters of credit outstanding	1,800,000		1,800,000
Letter of Credit | LOC for Security Deposit
Other Commitments [Line Items]
Letters of credit outstanding	$ 100,000		$ 100,000