Note 7 - Fair Value Measurements - Estimated Fair Value of Senior Notes (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Reported Value Measurement [Member]
Senior Notes, Carrying Amount	$ 987,967	$ 986,597
Estimate of Fair Value Measurement [Member]
Senior Notes, Fair Value	882,948	1,018,702
Senior Notes 5.625% Due February 2020 [Member] | Reported Value Measurement [Member]
Senior Notes, Carrying Amount	248,850	247,853
Senior Notes 5.625% Due February 2020 [Member] | Estimate of Fair Value Measurement [Member]
Senior Notes, Fair Value	253,413	261,991
Senior Notes 5.5% Due January 2024 [Member] | Reported Value Measurement [Member]
Senior Notes, Carrying Amount	248,789	248,585
Senior Notes 5.5% Due January 2024 [Member] | Estimate of Fair Value Measurement [Member]
Senior Notes, Fair Value	242,983	263,617
Senior Notes 6% Due January 2043 [Member] | Reported Value Measurement [Member]
Senior Notes, Carrying Amount	490,328	490,159
Senior Notes 6% Due January 2043 [Member] | Estimate of Fair Value Measurement [Member]
Senior Notes, Fair Value	$ 386,552	$ 493,094