Note 5 - Fair Value Measurements - Estimated Fair Value of Senior Notes (Details) (Parentheticals) - USD ($) $ in Millions	Mar. 31, 2020	Dec. 31, 2019
Senior Notes 5.625% Due February 2020 [Member]
Interest rate	5.625%	5.625%
Senior Notes 5.625% Due February 2020 [Member] | Reported Value Measurement [Member]
Interest rate	5.625%	5.625%
Senior Notes, face amount	$ 250	$ 250
Senior Notes 5.5% Due January 2024 [Member]
Interest rate	5.50%	5.50%
Senior Notes 5.5% Due January 2024 [Member] | Reported Value Measurement [Member]
Interest rate	5.50%	5.50%
Senior Notes, face amount	$ 250	$ 250
Senior Notes, 3 Point 85 Percent, Due January 2030 [Member]
Interest rate	3.85%	3.85%
Senior Notes, 3 Point 85 Percent, Due January 2030 [Member] | Reported Value Measurement [Member]
Interest rate	3.85%	3.85%
Senior Notes, face amount	$ 300	$ 300
Senior Notes 6% Due January 2043 [Member]
Interest rate	6.00%	6.00%
Senior Notes 6% Due January 2043 [Member] | Reported Value Measurement [Member]
Interest rate	6.00%	6.00%
Senior Notes, face amount	$ 500	$ 500