Stock-Based Compensation - Restricted Stock Units (Details) - Restricted stock units - USD ($) $ / shares in Units, $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vested in period, fair value	$ 7.7	$ 3.6
Number of Shares (in shares)
Unvested at beginning of period (in shares)	574,072
Granted (in shares)	2,295,322
Forfeited (in shares)	(47,733)
Vested (in shares)	(462,400)		0
Unvested at end of period (in shares)	2,359,261	574,072
Weighted- Average Grant Date Fair Value (in dollars per share)
Unvested at beginning of period (in dollars per share)	$ 14.51
Granted (in dollars per share)	24.84
Forfeited (in dollars per share)	22.85
Vested (in dollars per share)	16.73
Unvested at end of period (in dollars per share)	$ 23.95	$ 14.51
Unrecognized compensation expense	$ 41.6
Unrecognized compensation expense, period for recognition (years)	2 years 4 months 24 days
Minimum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Award vesting period (years)	3 years
Maximum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Award vesting period (years)	4 years
Cliff-vesting
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Award vesting period (years)	3 years
Cliff-vesting | Board of Directors
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Award vesting period (years)	1 year
Deferred vesting
Number of Shares (in shares)
Vested (in shares)	(417,163)
Weighted- Average Grant Date Fair Value (in dollars per share)
Accumulated number of shares recipients have elected to defer settlement (in shares)	554,621