Note 12 - Equity Compensation Plans (Details Textual) - USD ($) $ / shares in Units, $ in Millions			3 Months Ended		9 Months Ended
	Jul. 01, 2020	Jan. 15, 2020	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Proceeds from Stock Options Exercised			$ 26.0	$ 13.5	$ 68.3	$ 45.8
Nonqualified Stock Option, Contractual Term (Year)					10 years
Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Period (Year)					4 years
Share-based Compensation Arrangement by Share-based Payment Award, Options, Grants in Period, Gross (in shares)		882,749			924,810
Share-based Compensation Arrangement by Share-based Payment Award, Shares Issued in Period (in shares)	4,080
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Weighted Average Remaining Contractual Term (Year)					6 years 1 month 6 days
Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercisable, Weighted Average Remaining Contractual Term (Year)					4 years 9 months 18 days
Unrecognized Compensation Cost Related to Nonvested Share Based Compensation Arrangements			$ 94.3		$ 94.3
Share-based Payment Arrangement, Nonvested Award, Cost Not yet Recognized, Period for Recognition (Year)					2 years 6 months
Share-based Compensation Arrangement by Share-based Payment Award, Options, Vested in Period, Fair Value					$ 15.2	13.0
Share-based Payment Arrangement, Tranche One [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Options, Grants in Period, Gross (in shares)	3,570
Share-based Payment Arrangement, Tranche Two [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Period (Year)	12 months
Share-based Compensation Arrangement by Share-based Payment Award, Options, Grants in Period, Gross (in shares)	12,090
Share-based Compensation Arrangement by Share-based Payment Award, Award Requisite Service Period (Year)	1 year
Performance Shares [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Period (Year)		3 years			3 years
Share-based Compensation Arrangement by Share-based Payment Award, Number of Shares Per Award (in shares)					1
Share-based Compensation Arrangement by Share-based Payment Award, Target Levels					100.00%
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Grants in Period (in shares)		50,736			50,736
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Vested in Period, Fair Value					$ 6.2	3.0
Performance Shares [Member] | Minimum [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Target Levels					0.00%
Performance Shares [Member] | Maximum [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Target Levels					200.00%
Restricted Stock [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Grants in Period (in shares)		148,658			159,328
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Vested in Period, Fair Value					$ 17.1	$ 15.3
Restricted Stock [Member] | Graded Vesting Service Period [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Period (Year)		4 years
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Grants in Period (in shares)		141,725
Restricted Stock [Member] | Cliff Vesting Period [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Period (Year)		4 years
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Grants in Period (in shares)		6,933
Deferred Stock Units [Member] | Director [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Options, Grants in Period, Gross (in shares)	7,180
Employee Stock Purchase Plan [Member] | UK Sharesave Plan [Member]
Common Stock, Capital Shares Reserved for Future Issuance (in shares)			453,866		453,866
Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Period (Year)					3 years
Share-based Compensation Arrangement by Share-based Payment Award, Shares Issued in Period (in shares)					8,174	18,713
Share-based Compensation Arrangement by Share-based Payment Award, Discount from Market Price, Purchase Date					5.00%
Employee Stock Purchase Plan [Member] | US ESPP [Member]
Common Stock, Capital Shares Reserved for Future Issuance (in shares)			1,267,456		1,267,456
Share-based Compensation Arrangement by Share-based Payment Award, Shares Issued in Period (in shares)					25,312	23,321
Value of Common Stock Issued to Employees Per Share (in dollars per share)			$ 155.14	$ 138.02	$ 155.14	$ 138.02
The 2013 Incentive Plan [Member]
Common Stock, Capital Shares Reserved for Future Issuance (in shares)			3,099,195		3,099,195