Stock Option Summary (Details) - Employee Stock Option [Member] - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	9 Months Ended		12 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017	Dec. 31, 2016
[Line Items]
Outstanding, beginning balance (in shares)	956	959	959
Outstanding, beginning balance, weighted average exercise price per share (in dollars per share)	$ 47.10	$ 45.64	$ 45.64
Outstanding, weighted average remaining contractual life	6 years 7 months 6 days	6 years 6 months	6 years 3 months 18 days	6 years 7 months 6 days
Outstanding, beginning balance, aggregate intrinsic value	$ 21,965	$ 19,125	$ 19,125
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)	117	135
Granted, weighted average exercise price (in dollars per share)	$ 68.90	$ 65.94
Granted, weighted average grant date fair value	$ 24.55	$ 21.98
Exercised (in shares)	(151)	(36)
Exercised, weighted average exercise price (in dollars per share)	$ 38.98	$ 38.68
Exercised, aggregate intrinsic value	$ 4,509	$ 972
Forfeited (in share)	(3)	(35)
Forfeited, weighted average grant date fair value (in dollars per share)	$ 68.43	$ 47.38
Expired (in shares)	(66)	(47)
Expired, weighted average exercise price (in dollars per share)	$ 95.31	$ 67.43
Outstanding, ending balance (in shares)	895	976	956	959
Outstanding, ending balance, weighted average exercise price per share (in dollars per share)	$ 47.92	$ 47.60	$ 47.10	$ 45.64
Outstanding, weighted average remaining contractual life	6 years 7 months 6 days	6 years 6 months	6 years 3 months 18 days	6 years 7 months 6 days
Outstanding, ending balance, aggregate intrinsic value	$ 15,332	$ 30,320	$ 21,965	$ 19,125
Exercisable (in shares)	590
Exercisable, weighted average exercise price (in dollars per share)	$ 42.53
Exercisable, weighted average remaining contractual term	5 years 7 months 6 days
Exercisable, aggregate intrinsic value	$ 12,867
Expected to Vest [Member]
[Line Items]
Outstanding, weighted average remaining contractual life	8 years 6 months
Outstanding, ending balance (in shares)	305
Outstanding, ending balance, weighted average exercise price per share (in dollars per share)	$ 58.35
Outstanding, weighted average remaining contractual life	8 years 6 months
Outstanding, ending balance, aggregate intrinsic value	$ 2,466