Employee Benefits and Share-Based Compensation - Stock Option Activity (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018	Dec. 31, 2018
Weighted Average Exercise Price
Weighted-average remaining vesting period ( in years )			1 year 10 months 24 days
Aggregate Intrinsic Value
Unrecognized compensation cost of unvested stock options	$ 700		$ 700
2009 Plan | Employee stock option
Number of Shares
Outstanding at beginning of period (in shares)			3,748
Granted (in shares)			479
Exercised (in shares)			(525)
Expired (in shares)			(4)
Forfeited (in shares)			(178)
Outstanding at end of period (in shares)	3,520		3,520		3,748
Exercisable at end of period (in shares)	1,337		1,337
Vested and expected to vest at end of period (in shares)	3,520		3,520
Weighted Average Exercise Price
Outstanding beginning balance (in dollars per share)			$ 41.27
Granted (in dollars per share)			77.79
Exercised (in dollars per share)			32.19
Expired (in dollars per share)			40.26
Forfeited (in dollars per share)			44.86
Outstanding ending balance (in dollars per share)	$ 47.41		47.41		$ 41.27
Exercisable at end of period (in dollars per share)	32.48		32.48
Vested and expected to vest at end of period (in dollars per share)	$ 47.41		$ 47.41
Weighted-average remaining years, outstanding			7 years 8 months 12 days		7 years 7 months 6 days
Weighted-average remaining years, exercisable at end of period			6 years 3 months 18 days
Weighted-average remaining vesting period ( in years )			7 years 8 months 12 days
Aggregate Intrinsic Value
Aggregate intrinsic value, outstanding at end of period	$ 135,949		$ 135,949		$ 78,365
Aggregate intrinsic value, exercisable at end of period	71,596		71,596
Aggregate intrinsic value, vested and expected to vest at end of period	$ 135,949		$ 135,949
Weighted-average fair value (in dollars per share)	$ 24.40	$ 14.43	$ 24.20	$ 14.29
Intrinsic value of options exercised	$ 7,200	$ 6,500	$ 24,300	$ 8,500
Unrecognized compensation cost of unvested stock options	$ 33,200		$ 33,200
Weighted average period of compensation cost not yet recognized (in years)			2 years 9 months 18 days