Note 7 - Fair Value Measurements - Estimated Fair Value of Senior Notes (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Reported Value Measurement [Member]
Senior Notes, Carrying Amount	$ 989,422	$ 987,967
Estimate of Fair Value Measurement [Member]
Senior Notes, Fair Value	1,051,025	882,948
Senior Notes 5.625% Due February 2020 [Member] | Reported Value Measurement [Member]
Senior Notes, Carrying Amount	249,909	248,850
Senior Notes 5.625% Due February 2020 [Member] | Estimate of Fair Value Measurement [Member]
Senior Notes, Fair Value	250,400	253,413
Senior Notes 5.5% Due January 2024 [Member] | Reported Value Measurement [Member]
Senior Notes, Carrying Amount	249,005	248,789
Senior Notes 5.5% Due January 2024 [Member] | Estimate of Fair Value Measurement [Member]
Senior Notes, Fair Value	272,083	242,983
Senior Notes 6% Due January 2043 [Member] | Reported Value Measurement [Member]
Senior Notes, Carrying Amount	490,508	490,328
Senior Notes 6% Due January 2043 [Member] | Estimate of Fair Value Measurement [Member]
Senior Notes, Fair Value	$ 528,542	$ 386,552