Stock Option Summary (Details) - Employee Stock Option [Member] - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Dec. 31, 2018	Dec. 31, 2017
[Line Items]
Outstanding, beginning balance (in shares)	895	956	956
Outstanding, beginning balance, weighted average exercise price per share (in dollars per share)	$ 47.93	$ 47.10	$ 47.10
Outstanding, weighted average remaining contractual life	6 years	6 years 6 months	6 years 2 months 12 days	6 years 3 months 18 days
Outstanding, beginning balance, aggregate intrinsic value	$ 4,806	$ 21,965	$ 21,965
Shares converted upon acquisition (in shares)		42
Shares converted upon acquisition, weighted average exercise price (in dollars per share)		$ 51.86
Shares converted upon acquisition, weighted average grant date fair value		$ 14.86
Granted (in shares)		101
Granted, weighted average exercise price (in dollars per share)		$ 69.30
Granted, weighted average grant date fair value		$ 24.83
Exercised (in shares)	(20)	(62)
Exercised, weighted average exercise price (in dollars per share)	$ 44.99	$ 40.31
Exercised, aggregate intrinsic value	$ 211	$ 2,104
Forfeited (in share)	(7)	(3)
Forfeited, weighted average grant date fair value (in dollars per share)	$ 67.53	$ 72.25
Expired (in shares)		(7)
Expired, weighted average exercise price (in dollars per share)		$ 95.96
Outstanding, ending balance (in shares)	868	1,027	895	956
Outstanding, ending balance, weighted average exercise price per share (in dollars per share)	$ 47.83	$ 49.49	$ 47.93	$ 47.10
Outstanding, weighted average remaining contractual life	6 years	6 years 6 months	6 years 2 months 12 days	6 years 3 months 18 days
Outstanding, ending balance, aggregate intrinsic value	$ 4,402	$ 24,051	$ 4,806	$ 21,965
Exercisable (in shares)	734
Exercisable, weighted average exercise price (in dollars per share)	$ 44.46
Exercisable, weighted average remaining contractual term	5 years 6 months
Exercisable, aggregate intrinsic value	$ 4,402
Expected to Vest [Member]
[Line Items]
Outstanding, weighted average remaining contractual life	8 years 7 months 6 days
Outstanding, ending balance (in shares)	134
Outstanding, ending balance, weighted average exercise price per share (in dollars per share)	$ 66.25
Outstanding, weighted average remaining contractual life	8 years 7 months 6 days
Outstanding, ending balance, aggregate intrinsic value	$ 0