Note 7 - Fair Value Measurements - Estimated Fair Value of Senior Notes (Details) (Parentheticals) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017	Oct. 16, 2017
Senior Notes 5.625% Due February 2020 [Member]
Senior Notes, interest rate	5.625%	5.625%
Senior Notes, face amount	$ 250.0	$ 250.0
Senior Notes 5.625% Due February 2020 [Member] | Reported Value Measurement [Member]
Senior Notes, interest rate	5.625%	5.625%
Senior Notes, face amount	$ 250.0	$ 250.0
Senior Notes 5.625% Due February 2020 [Member] | Estimate of Fair Value Measurement [Member]
Senior Notes, interest rate	5.625%	5.625%
Senior Notes, face amount	$ 250.0	$ 250.0
Senior Notes 5.5% Due January 2024 [Member]
Senior Notes, interest rate	5.50%	5.50%
Senior Notes, face amount	$ 250.0	$ 250.0
Senior Notes 5.5% Due January 2024 [Member] | Reported Value Measurement [Member]
Senior Notes, interest rate	5.50%	5.50%
Senior Notes, face amount	$ 250.0	$ 250.0
Senior Notes 5.5% Due January 2024 [Member] | Estimate of Fair Value Measurement [Member]
Senior Notes, interest rate	5.50%	5.50%
Senior Notes, face amount	$ 250.0	$ 250.0
Senior Notes 6% Due January 2043 [Member]
Senior Notes, interest rate	6.00%	6.00%
Senior Notes, face amount	$ 500.0	$ 500.0	$ 500.0
Senior Notes 6% Due January 2043 [Member] | Reported Value Measurement [Member]
Senior Notes, interest rate	6.00%	6.00%
Senior Notes, face amount	$ 500.0	$ 500.0
Senior Notes 6% Due January 2043 [Member] | Estimate of Fair Value Measurement [Member]
Senior Notes, interest rate	6.00%	6.00%
Senior Notes, face amount	$ 500.0	$ 500.0