Note 14 - Senior Notes - Carrying Amount of Senior Notes (Details) (Parentheticals)	3 Months Ended	12 Months Ended
Mar. 31, 2018	Dec. 31, 2017
Senior Notes 5.625% Due February 2020 [Member]
Senior Notes, interest rate	5.625%	5.625%
Maturity date	Feb. 29, 2020	Feb. 29, 2020
Senior Notes 5.5% Due January 2024 [Member]
Senior Notes, interest rate	5.50%	5.50%
Maturity date	Jan. 31, 2024	Jan. 31, 2024
Senior Notes 6% Due January 2043 [Member]
Senior Notes, interest rate	6.00%	6.00%
Maturity date	Jan. 31, 2043	Jan. 31, 2043

Date when the debt instrument is scheduled to be fully repaid, in CCYY-MM-DD format.

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.22(a)(2))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

Reference 2: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (bbb)(2)
-URI http://asc.fasb.org/extlink&oid=99377789&loc=d3e19207-110258

Name:	us-gaap_DebtInstrumentMaturityDate
Namespace Prefix:	us-gaap_
Data Type:	xbrli:dateItemType
Balance Type:	na
Period Type:	duration