Note 4 - Derivative Financial Instruments (Details Textual) $ in Millions	Dec. 31, 2015 USD ($)
Senior Notes [Member] | Senior Notes, First Issuance [Member]
Unsecured Debt	$ 250.0
Debt Instrument, Interest Rate, Stated Percentage	2.40%
Senior Notes [Member] | Senior Notes, Third Issuance [Member]
Unsecured Debt	$ 350.0
Debt Instrument, Interest Rate, Stated Percentage	3.30%
Senior Notes [Member] | Senior Notes, Second Issuance [Member]
Unsecured Debt	$ 250.0
Debt Instrument, Interest Rate, Stated Percentage	3.85%
Senior Notes, Third Issuance [Member] | Interest Rate Swap [Member] | Fair Value Hedging [Member] | London Interbank Offered Rate (LIBOR) [Member]
Derivative, Variable Interest Rate	1.72%
Senior Notes, Second Issuance [Member] | Interest Rate Swap [Member] | Fair Value Hedging [Member] | London Interbank Offered Rate (LIBOR) [Member]
Derivative, Variable Interest Rate	1.36%