Note 5 - Fair Value Measurements - Estimated Fair Value of Senior Notes (Details) - USD ($)	Mar. 31, 2020	Dec. 31, 2019
Senior Notes, Carrying Amount	$ 1,036,900,000	$ 989,422,000
Reported Value Measurement [Member]
Senior Notes, Carrying Amount	1,036,900,000	989,422,000
Estimate of Fair Value Measurement [Member]
Senior Notes, Fair Value	965,750,000	1,051,025,000
Senior Notes 5.625% Due February 2020 [Member]
Senior Notes, Carrying Amount	0	249,909,000
Senior Notes 5.625% Due February 2020 [Member] | Reported Value Measurement [Member]
Senior Notes, Carrying Amount	0	249,909,000
Senior Notes 5.625% Due February 2020 [Member] | Estimate of Fair Value Measurement [Member]
Senior Notes, Fair Value	0	250,400,000
Senior Notes 5.5% Due January 2024 [Member]
Senior Notes, Carrying Amount	249,060,000	249,005,000
Senior Notes 5.5% Due January 2024 [Member] | Reported Value Measurement [Member]
Senior Notes, Carrying Amount	249,060,000	249,005,000
Senior Notes 5.5% Due January 2024 [Member] | Estimate of Fair Value Measurement [Member]
Senior Notes, Fair Value	228,750,000	272,083,000
Senior Notes, 3 Point 85 Percent, Due January 2030 [Member]
Senior Notes, Carrying Amount	297,285,000	0
Senior Notes, 3 Point 85 Percent, Due January 2030 [Member] | Reported Value Measurement [Member]
Senior Notes, Carrying Amount	297,285,000	0
Senior Notes, 3 Point 85 Percent, Due January 2030 [Member] | Estimate of Fair Value Measurement [Member]
Senior Notes, Fair Value	270,750,000	0
Senior Notes 6% Due January 2043 [Member]
Senior Notes, Carrying Amount	490,555	490,508
Senior Notes 6% Due January 2043 [Member] | Reported Value Measurement [Member]
Senior Notes, Carrying Amount	490,555,000	490,508,000
Senior Notes 6% Due January 2043 [Member] | Estimate of Fair Value Measurement [Member]
Senior Notes, Fair Value	$ 466,250,000	$ 528,542,000