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Note 12 - Equity Compensation Plans (Tables)
3 Months Ended
Mar. 31, 2021
Notes Tables  
Share-based Payment Arrangement, Option, Activity [Table Text Block]
  

Stock Option

  

Restricted Stock

  

PSU

 
  

Number of Shares

  

Weighted Average Exercise Price

  

Aggregate Intrinsic Value

  

Number of Shares

  

Weighted Average Grant Date Fair Value Per Share

  

Number of Shares

  

Weighted Average Grant Date Fair Value Per Share

 
          

(in millions)

                 

Outstanding at December 31, 2020

  5,611,777  $98.28  $613.4   390,054  $131.63   145,609  $170.75 

Granted

  709,811  $190.10       135,380  $190.10   59,144  $210.07 

Dividend reinvestment

    $         $   269   N/A 

Exercised or lapsed

  (115,284) $64.73  $13.5   (36,637) $158.23   (42,610) $140.70 

Canceled, expired or forfeited

  (20,245) $136.35       (3,216) $141.28        

Outstanding at March 31, 2021

  6,186,059  $109.31  $416.8   485,581  $145.47   162,412  $192.99 

Exercisable at March 31, 2021

  3,588,921  $82.06  $339.6                 

Exercisable at December 31, 2020

  3,494,164  $76.84  $456.9                 

Nonvested at March 31, 2021

  2,597,138           485,581       162,412     

Expected to vest at March 31, 2021

  2,296,123           432,880       115,599 (1)    
Schedule of Share-based Payment Award, Stock Options, Valuation Assumptions [Table Text Block]
  

2021

  

2020

 
Option pricing model  Black-Scholes   Black-Scholes 

Expected volatility

  23.60%  18.17%

Risk-free interest rate

  0.37%  1.59%

Expected term in years

  4.3   4.3 

Dividend yield

  0.63%  0.71%

Weighted average grant date fair value per stock option

 $36.09  $25.49