Note 12 - Notes Payable (Details Textual) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Gain (Loss) on Extinguishment of Debt, Total	$ 0	$ (12,762)	$ (1,753)
Notes Payable, Total	4,831,759	4,381,456
Debt Issuance Costs, Net, Total	1,500
Debt Covenants Dividend Payment Restriction Maximum Payment	26,000
Long-term Line of Credit, Total	200,000
Senior Unsecured Note [Member]
Gain (Loss) on Extinguishment of Debt, Total		(12,800)
Notes Payable, Total		$ 484,900
Unsecured Debt [Member]
Debt Issuance Costs, Net, Total	$ 53,100
Revolving Credit Facility [Member]
Debt Instrument, Interest Rate, Effective Percentage	2.64%	3.31%
Line of Credit Facility, Current Borrowing Capacity	$ 2,250,000
Line of Credit Facility Sub-limit	500,000
Revolving Credit Facility [Member] | Credit Facility Accordion Feature [Member]
Line of Credit Facility, Maximum Borrowing Capacity	$ 2,750,000
Revolving Credit Facility [Member] | London Interbank Offered Rate (LIBOR) [Member]
Debt Instrument, Basis Spread on Variable Rate	0.875%
Letter of Credit [Member]
Long-term Line of Credit, Total		$ 300