Note 6 - Fair Value Measurements - Estimated Fair Value of Senior Notes (Details) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Carrying amount	$ 842,532	$ 841,646
Fair value	878,786	821,498
Senior Notes 5.625% Due February 2020 [Member]
Carrying amount	247,613	246,915
Fair value	268,113	265,611
Senior Notes 5.5% Due January 2024 [Member]
Carrying amount	248,535	248,391
Fair value	270,250	258,800
Senior Notes 6% Due January 2043 [Member]
Carrying amount	346,384	346,340
Fair value	$ 340,423	$ 297,087

Amount, before unamortized (discount) premium and debt issuance costs, of long-term debt. Includes, but is not limited to, notes payable, bonds payable, commercial loans, mortgage loans, convertible debt, subordinated debt and other types of debt.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02(22))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-03(16))
-URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03(16))
-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

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Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant