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, 2018	Mar. 31, 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 6 months	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	121
Granted, Weighted Average Exercise Price Per Share	$ 69.30	$ 65.55
Granted, Weighted Average Grant Date Fair Value	$ 24.83	$ 22.01
Exercised, Number	(62)	(5)
Exercised, Weighted Average Exercise Price Per Share	$ 40.31	$ 35.29
Exercised, Aggregate Intrinsic Value	$ 2,104	$ 120
Share-based Compensation Arrangement by Share-based Payment Award, Options, Nonvested Options Forfeited, Number of Shares	(3)
Share-based Compensation Arrangement by Share-based Payment Award, Options, Nonvested Options Forfeited, Weighted Average Grant Date Fair Value	$ 72.25
Expired, Number	(7)
Expired, Weighted Average Exercise Price Per Share	$ 95.96
Outstanding, ending balance, Number	1,027	1,075	956	959
Outstanding, ending balance, Weighted Average Exercise Price Per Share	$ 49.49	$ 47.92	$ 47.10	$ 45.64
Outstanding, ending balance, Aggregate Intrinsic Value	$ 24,051	$ 17,236	$ 21,965	$ 19,125
Exercisable, Number	716
Exercisable, Weighted Average Exercise Price Per Share	$ 45.96
Exercisable, Weighted Average Remaining Contractual Term	5 years 6 months
Exercisable, Aggregate Intrinsic Value	$ 19,725
Expected to Vest [Member]
[Line Items]
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Weighted Average Remaining Contractual Term	8 years 10 months 24 days
Outstanding, ending balance, Number	308
Outstanding, ending balance, Weighted Average Exercise Price Per Share	$ 57.74
Outstanding, ending balance, Aggregate Intrinsic Value	$ 4,255