Note 12 - Revolving Credit Facility and Letter of Credit Facilities (Details Textual) $ in Thousands	3 Months Ended
	Mar. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)
Cash Collateral Deposits	$ 26,600
Long-term Line of Credit
Four Committed Letter of Credit [Member]
Letters of Credit Outstanding, Amount	$ 26,100
London Interbank Offered Rate (LIBOR) [Member]
Debt Instrument, Basis Spread on Variable Rate	0.983%
Prime Rate [Member]
Debt Instrument, Basis Spread on Variable Rate	4.00%
Long-term Debt, Percentage Bearing Fixed Interest, Percentage Rate	0.75%
Unsecured Revolving Credit Facility [Member]
Line of Credit Facility, Current Borrowing Capacity	$ 750,000
Line of Credit Facility, Maximum Borrowing Capacity	1,200,000
Line of Credit Facility Covenant, Minimum Tangible Net Worth Requirement	$ 1,650,000
Line of Credit Facility Covenant, Maximum Leverage Ratio	2
Line of Credit Facility Covenant, Minimum Interest Coverage Ratio	1.25
Letters of Credit Outstanding, Amount	$ 106,500
Line of Credit Facility, Remaining Borrowing Capacity	643,500
Long-term Line of Credit	$ 0
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	$ 350,000
Letter of Credit [Member] | Four Committed Letter of Credit [Member]
Line of Credit Facility, Maximum Borrowing Capacity	$ 40,200