Note 5 - Derivative Financial Instruments (Details Textual) $ in Millions	Sep. 30, 2017 USD ($)
Senior Notes, First Issuance [Member] | Fair Value Hedging [Member] | Interest Rate Swap [Member] | London Interbank Offered Rate (LIBOR) [Member]
Derivative, Variable Interest Rate	2.17%
Senior Notes, Third Issuance [Member] | Fair Value Hedging [Member] | Interest Rate Swap [Member] | London Interbank Offered Rate (LIBOR) [Member]
Derivative, Variable Interest Rate	2.67%
Senior Notes [Member] | Senior Notes, First Issuance [Member]
Unsecured Debt	$ 250
Debt Instrument, Interest Rate, Stated Percentage	2.40%
Senior Notes [Member] | Senior Notes, Third Issuance [Member]
Unsecured Debt	$ 350
Debt Instrument, Interest Rate, Stated Percentage	3.30%