Stock-Based Compensation Plans - Options (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended	9 Months Ended	12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Options
Options outstanding at beginning of period (in shares)			18,091,793
Grants (in shares)			886,115
Exercises (in shares)			(984,585)
Forfeitures and adjustments (in shares)			(253,579)
Options outstanding at end of period (in shares)	18,091,793		17,739,744	18,091,793
Vested options outstanding			8,061,312	0
Weighted Average Grant Date Fair Value
Options outstanding at beginning of period (in dollars per share)			$ 1.85
Grants (in dollars per share)			8.83
Exercises (in dollars per share)			1.80
Forfeitures and adjustments (in dollars per share)			2.67
Options outstanding at end of period (in dollars per share)	$ 1.85		2.19	$ 1.85
Vested options outstanding (in dollars per share)			$ 1.80
Options exercised, intrinsic value			$ 21,300
Exercisable	0			0
Compensation expense	$ 9,400	$ 15,900	50,700		$ 26,100
Options
Weighted Average Grant Date Fair Value
Compensation expense			24,432
Initial Public Offering
Weighted Average Grant Date Fair Value
Compensation expense			$ 18,900