Note 12 - Revolving Credit Facility and Letter of Credit Facilities (Details Textual) $ in Millions	9 Months Ended
	Sep. 30, 2017 USD ($)
Cash Collateral Deposits	$ 22.8
London Interbank Offered Rate (LIBOR) [Member]
Debt Instrument, Basis Spread on Variable Rate	1.24%
Prime Rate [Member]
Debt Instrument, Basis Spread on Variable Rate	4.25%
Long-term Debt, Percentage Bearing Fixed Interest, Percentage Rate	0.75%
Unsecured Revolving Credit Facility [Member]
Line of Credit Facility, Current Borrowing Capacity	$ 750.0
Line of Credit Facility, Maximum Borrowing Capacity	1,200.0
Line of Credit Facility Covenant, Minimum Tangible Net Worth Requirement	$ 1,650.0
Line of Credit Facility Covenant, Maximum Leverage Ratio	2
Line of Credit Facility Covenant, Minimum Interest Coverage Ratio	1.25
Long-term Line of Credit	$ 295.6
Letters of Credit Outstanding, Amount	119.2
Line of Credit Facility, Remaining Borrowing Capacity	$ 335.2
Unsecured Revolving Credit Facility [Member] | London Interbank Offered Rate (LIBOR) [Member]
Long-term Debt, Percentage Bearing Fixed Interest, Percentage Rate	1.75%
Letter of Credit [Member]
Line of Credit Facility, Current Borrowing Capacity	$ 350.0
Letter of Credit [Member] | Four Committed Letter of Credit [Member]
Line of Credit Facility, Maximum Borrowing Capacity	48.0
Letters of Credit Outstanding, Amount	$ 22.4