Note 9 - Notes and Mortgages Payable - Senior Unsecured Notes Issuance (Details) $ in Millions	9 Months Ended
Sep. 30, 2017 USD ($)
Senior Unsecured Notes 1 [Member]
Maturity Date	Apr-27
Amount Issued	$ 400.0
Interest Rate	3.80%
Senior Unsecured Notes 2 [Member]
Maturity Date	Feb-25
Amount Issued	$ 500.0
Interest Rate	3.30%
Senior Unsecured Notes 3 [Member]
Maturity Date	Sept-47
Amount Issued	$ 350.0
Interest Rate	4.45%

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

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-Section 55
-Paragraph 8
-URI http://asc.fasb.org/extlink&oid=64930739&loc=d3e28878-108400

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-Section 45
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=64930785&loc=d3e28551-108399

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