Note 6 - Fair Value Measurements - Estimated Fair Value of Senior Notes (Details) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Senior Notes, Carrying Amount	$ 988,683	$ 987,967
Reported Value Measurement [Member]
Senior Notes, Carrying Amount	988,683	987,967
Estimate of Fair Value Measurement [Member]
Senior Notes, Fair Value	987,496	882,948
Senior Notes 5.625% Due February 2020 [Member]
Senior Notes, Carrying Amount	249,371	248,850
Senior Notes 5.625% Due February 2020 [Member] | Reported Value Measurement [Member]
Senior Notes, Carrying Amount	249,371	248,850
Senior Notes 5.625% Due February 2020 [Member] | Estimate of Fair Value Measurement [Member]
Senior Notes, Fair Value	253,175	253,413
Senior Notes 5.5% Due January 2024 [Member]
Senior Notes, Carrying Amount	248,895	248,789
Senior Notes 5.5% Due January 2024 [Member] | Reported Value Measurement [Member]
Senior Notes, Carrying Amount	248,895	248,789
Senior Notes 5.5% Due January 2024 [Member] | Estimate of Fair Value Measurement [Member]
Senior Notes, Fair Value	267,121	242,983
Senior Notes 6% Due January 2043 [Member]
Senior Notes, Carrying Amount	490,417	490,328
Senior Notes 6% Due January 2043 [Member] | Reported Value Measurement [Member]
Senior Notes, Carrying Amount	490,417	490,328
Senior Notes 6% Due January 2043 [Member] | Estimate of Fair Value Measurement [Member]
Senior Notes, Fair Value	$ 467,200	$ 386,552