Note 13 - Notes Payable (Details Textual) CAD in Millions				1 Months Ended					12 Months Ended
		Aug. 26, 2016 USD ($)	Aug. 26, 2016 CAD	Feb. 24, 2017 USD ($)	Jan. 31, 2017 USD ($)	Nov. 30, 2016 USD ($)	Aug. 31, 2016 USD ($)	Jan. 31, 2015 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)
Gain (Loss) on Extinguishment of Debt										$ (45,674,000)
Debt Covenants Dividend Payment Restriction Maximum Payment										26,000,000
Line of Credit Facility Sub-limit										500,000,000
Long-term Line of Credit										25,000,000
Revolving Credit Facility [Member]
Line of Credit Facility, Current Borrowing Capacity										1,750,000,000
Line of Credit Facility, Maximum Borrowing Capacity										2,250,000,000
Letter of Credit [Member]
Line of Credit Facility, Current Borrowing Capacity										1,750,000,000
Long-term Line of Credit										$ 700,000
Subsequent Event [Member]
Line of Credit Facility Sub-limit				$ 500,000,000
Subsequent Event [Member] | Revolving Credit Facility [Member]
Line of Credit Facility, Current Borrowing Capacity				2,250,000,000
Subsequent Event [Member] | Revolving Credit Facility [Member] | Credit Facility Accordion Feature [Member]
Line of Credit Facility, Maximum Borrowing Capacity				$ 2,750,000,000
London Interbank Offered Rate (LIBOR) [Member] | Subsequent Event [Member] | Revolving Credit Facility [Member]
Debt Instrument, Basis Spread on Variable Rate				0.875%
Term Loan [Member]
Debt Instrument, Interest Rate, Stated Percentage										1.60%	1.37%
Repayments of Long-term Debt						$ 400,000,000		$ 400,000,000
Unsecured Debt								$ 650,000,000		$ 250,000,000
Term Loan [Member] | Subsequent Event [Member]
Repayments of Long-term Debt					$ 250,000,000				$ 250,000,000
Term Loan [Member] | London Interbank Offered Rate (LIBOR) [Member]
Debt Instrument, Basis Spread on Variable Rate										0.95%		1.05%
Line of Credit [Member]
Debt Instrument, Interest Rate, Stated Percentage										1.67%	1.35%
Line of Credit [Member] | London Interbank Offered Rate (LIBOR) [Member]
Debt Instrument, Basis Spread on Variable Rate										0.925%
Unsecured Debt [Member]
Debt, Weighted Average Interest Rate										3.58%
Debt Issuance Costs, Net										$ 47,700,000
Long-term Debt, Maturities, Repayments of Principal in Next Twelve Months										250,000,000
Long-term Debt, Maturities, Repayments of Principal in Year Two										325,000,000
Long-term Debt, Maturities, Repayments of Principal in Year Three										300,000,000
Long-term Debt, Maturities, Repayments of Principal in Year Four										0
Long-term Debt, Maturities, Repayments of Principal in Year Five										500,000,000
Long-term Debt, Maturities, Repayments of Principal after Year Five										2,600,000,000
Gain (Loss) on Extinguishment of Debt										$ (10,200,000)
Canadian Notes Payable, Mature in April 2018 [Member]
Debt Instrument, Interest Rate, Stated Percentage		5.99%	5.99%
Repayments of Long-term Debt | CAD			CAD 150.0
Canadian Notes Payable, Mature in August 2020 [Member]
Debt Instrument, Interest Rate, Stated Percentage		3.855%	3.855%
Repayments of Long-term Debt | CAD			CAD 200.0
Canadian Notes Payable [Member]
Repayments of Long-term Debt	[1]						$ 270,900,000
Gain (Loss) on Extinguishment of Debt		$ (26,300,000)	CAD (34.1)

[1] On August 26, 2016, the redemption date, the Company repaid (i) its Canadian denominated ("CAD") $150.0 million 5.99% notes, which were scheduled to mature in April 2018 and (ii) its CAD $200.0 million 3.855% notes, which were scheduled to mature in August 2020. The Company recorded aggregate early extinguishment of debt charges of CAD $34.1 million (USD $26.3 million) resulting from the early repayment of these notes.