Note 6 - Fair Value Measurements - Estimated Fair Value of Senior Notes (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Senior Notes, Carrying Amount	$ 988,322	$ 987,967
Reported Value Measurement [Member]
Senior Notes, Carrying Amount	988,322	987,967
Estimate of Fair Value Measurement [Member]
Senior Notes, Fair Value	946,900	882,948
Senior Notes 5.625% Due February 2020 [Member]
Senior Notes, Carrying Amount	249,109	248,850
Senior Notes 5.625% Due February 2020 [Member] | Reported Value Measurement [Member]
Senior Notes, Carrying Amount	249,109	248,850
Senior Notes 5.625% Due February 2020 [Member] | Estimate of Fair Value Measurement [Member]
Senior Notes, Fair Value	254,304	253,413
Senior Notes 5.5% Due January 2024 [Member]
Senior Notes, Carrying Amount	248,841	248,789
Senior Notes 5.5% Due January 2024 [Member] | Reported Value Measurement [Member]
Senior Notes, Carrying Amount	248,841	248,789
Senior Notes 5.5% Due January 2024 [Member] | Estimate of Fair Value Measurement [Member]
Senior Notes, Fair Value	258,479	242,983
Senior Notes 6% Due January 2043 [Member]
Senior Notes, Carrying Amount	490,372	490,328
Senior Notes 6% Due January 2043 [Member] | Reported Value Measurement [Member]
Senior Notes, Carrying Amount	490,372	490,328
Senior Notes 6% Due January 2043 [Member] | Estimate of Fair Value Measurement [Member]
Senior Notes, Fair Value	$ 434,117	$ 386,552