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Note 4 - Fair Value Measurements - Long-term Debt (Details) - Fair Value, Inputs, Level 2 [Member] - USD ($)
$ in Millions
Mar. 31, 2021
Dec. 31, 2020
Reported Value Measurement [Member]    
Senior notes (Note 10) $ 3,141.1 $ 3,140.8
Estimate of Fair Value Measurement [Member]    
Senior notes (Note 10) $ 3,426.9 $ 3,652.2