Note 6 - Investments Securities Lending Activity (Details)		9 Months Ended	12 Months Ended
		Sep. 30, 2017	Dec. 31, 2016 USD ($) transaction	Dec. 31, 2017 USD ($)
Securities Financing Transaction [Line Items]
Securities Lending Rate of Collateral Required		1.02
Securities Loaned, Asset				$ 27,964,000
Securities Held as Collateral, at Fair Value				0
Securities Received as Collateral			$ 0	19,357,000
Number of Securities Lending Transactions | transaction			0
Repurchase Agreements, Securities Lending Transactions, and Repurchase-to-Maturity Transactions Accounted for as Secured Borrowings, Description of Potential Risks		The Borrower generally may return the loaned securities to us at any time, which would require us to return the collateral within the standard settlement period for the loaned securities on the principal exchange or market in which the securities are traded. We manage this liquidity risk associated with cash collateral by maintaining the cash collateral in a short-term money-market fund with daily availability. The credit risk under these programs is reduced by the amounts of collateral received. On a daily basis, the value of the underlying securities that we have loaned to the Borrowers is compared to the value of cash and securities collateral we received from the Borrowers, and additional cash or securities are requested or returned, as applicable. In addition, we are indemnified against counterparty credit risk by the intermediary.
Securities Financing Transaction, Fair Value [Member]
Securities Financing Transaction [Line Items]
Securities Lending Rate of Collateral Required		1.00
Securities Loaned, Asset	[1]			27,964,000
Securities Borrowed, Fair Value of Collateral	[2]			9,342,000
Securities Received as Collateral	[3]			19,357,000
Securities Financing Transaction, Fair Value [Member] | Equity securities
Securities Financing Transaction [Line Items]
Securities Loaned, Asset	[1]			13,235,000
Securities Financing Transaction, Fair Value [Member] | Corporate bonds and notes
Securities Financing Transaction [Line Items]
Securities Loaned, Asset	[1]			13,862,000
Securities Financing Transaction, Fair Value [Member] | Foreign Government Debt Securities [Member]
Securities Financing Transaction [Line Items]
Securities Loaned, Asset	[1]			867,000
Securities Financing Transaction, Cost [Member]
Securities Financing Transaction [Line Items]
Securities Loaned, Asset				$ 27,846,000
Foreign Government Debt Securities [Member]
Securities Financing Transaction [Line Items]
Securities Lending Rate of Collateral Required		1.05

[1] Our securities loaned under securities lending agreements are included at fair value within other assets on our consolidated balance sheets. All of our securities lending agreements are classified as overnight and revolving. None of the amounts are subject to offsetting.

[2] Securities collateral on deposit with us from Borrowers may not be transferred or re-pledged unless the Borrower is in default, and is therefore not reflected in our consolidated financial statements.

[3] All cash collateral received has been reinvested in accordance with the securities lending agreements and is included in short-term investments. Amounts payable on the return of cash collateral under securities lending agreements are included within other liabilities on our consolidated balance sheets.