Note 16 - Preferred Stock, Common Stock and Convertible Unit Transactions - Preferred Stock Issued (Details) - USD ($) $ / shares in Units, $ in Millions		1 Months Ended	12 Months Ended
		Jan. 31, 2018	Dec. 31, 2017
Series L Preferred Stock [Member]
Depositary Shares Issued (in shares)			9,000,000
Net Proceeds, After Expenses			$ 218.1
Offering/ Redemption Price (in dollars per share)			$ 25
Series M Preferred Stock [Member]
Depositary Shares Issued (in shares)		1,380,000	9,200,000	[1]
Net Proceeds, After Expenses	[1]		$ 222.8
Offering/ Redemption Price (in dollars per share)	[1]		$ 25

[1] Additionally, during January 2018, the underwriting financial institutions for the Class M issuance elected to exercise the over-allotment option and as a result, the Company issued an additional 1,380,000 Class M Depositary Shares, each representing a one-thousandth fractional interest in a share of the Company's 5.250% Class M Cumulative Redeemable Preferred Stock, $1.00 par value per share. The Company received net proceeds before expenses of $33.4 million from this offering.