Note 6 - Fair Value Measurements - Estimated Fair Value of Senior Notes (Details) (Parentheticals) - USD ($) $ in Millions	Mar. 31, 2018	Dec. 31, 2017
Senior Notes 5.625% Due February 2020 [Member]
Senior Notes, interest rate	5.625%	5.625%
Senior Notes, face amount		$ 250
Senior Notes 5.5% Due January 2024 [Member]
Senior Notes, interest rate	5.50%	5.50%
Senior Notes, face amount		$ 250
Senior Notes 6% Due January 2043 [Member]
Senior Notes, interest rate	6.00%	6.00%
Senior Notes, face amount		$ 500

Face (par) amount of debt instrument at time of issuance.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-Section 45
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=114775744&loc=d3e28551-108399

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-Section 55
-Paragraph 8
-URI http://asc.fasb.org/extlink&oid=114775985&loc=d3e28878-108400

Name:	us-gaap_DebtInstrumentFaceAmount
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant