Stock Option Summary (Details) - Employee Stock Option [Member] - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	6 Months Ended		12 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017	Dec. 31, 2016
[Line Items]
Outstanding, beginning balance, Number	956	959	959
Outstanding, beginning balance, Weighted Average Exercise Price Per Share	$ 47.10	$ 45.64	$ 45.64
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Weighted Average Remaining Contractual Term	6 years 8 months 12 days	6 years 8 months 12 days	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, number	42
Shares converted upon acquisition, Weighted Average Exercise Price	$ 51.86
Shares converted upon acquisition, Weighted Average Grant Date Fair Value	$ 14.86
Options granted, number	101	132
Granted, Weighted Average Exercise Price Per Share	$ 69.30	$ 65.80
Granted, Weighted Average Grant Date Fair Value	$ 24.83	$ 21.99
Exercised, Number	(87)	(34)
Exercised, Weighted Average Exercise Price Per Share	$ 40.08	$ 37.58
Exercised, Aggregate Intrinsic Value	$ 2,779	$ 933
Share-based Compensation Arrangement by Share-based Payment Award, Options, Nonvested Options Forfeited, Number of Shares	(3)	(35)
Share-based Compensation Arrangement by Share-based Payment Award, Options, Nonvested Options Forfeited, Weighted Average Grant Date Fair Value	$ 68.43	$ 47.38
Expired, Number	(66)	(47)
Expired, Weighted Average Exercise Price Per Share	$ 95.33	$ 67.43
Outstanding, ending balance, Number	943	975	956	959
Outstanding, ending balance, Weighted Average Exercise Price Per Share	$ 46.92	$ 47.54	$ 47.10	$ 45.64
Outstanding, ending balance, Aggregate Intrinsic Value	$ 14,066	$ 21,680	$ 21,965	$ 19,125
Exercisable, Number	648
Exercisable, Weighted Average Exercise Price Per Share	$ 41.97
Exercisable, Weighted Average Remaining Contractual Term	5 years 9 months 18 days
Exercisable, Aggregate Intrinsic Value	$ 12,006
Expected to Vest [Member]
[Line Items]
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Weighted Average Remaining Contractual Term	8 years 8 months 12 days
Outstanding, ending balance, Number	295
Outstanding, ending balance, Weighted Average Exercise Price Per Share	$ 57.82
Outstanding, ending balance, Aggregate Intrinsic Value	$ 2,061