Commitments and Contingencies (Employee Benefit Plan and Letters of Credit Narrative) (Details) - USD ($)	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Other Commitments [Line Items]
Employer matching contribution, percent of match		50.00%
Employer matching contribution percent of eligible compensation		5.00%
Employer contribution limit		$ 6,000
Employer contribution vesting period		5 years
Employer matching contributions	$ 1,000,000	$ 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	1,300,000
Letter of Credit | LOC for Security Deposit
Other Commitments [Line Items]
Letters of credit outstanding	$ 100,000