Note 5 - Investments Securities Lending Agreements (Details)		9 Months Ended
		Sep. 30, 2018 USD ($)	Dec. 31, 2017 USD ($)
Securities Financing Transaction [Line Items]
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.
Loaned securities		$ 34,364,000	$ 27,964,000
Cash Collateral for Borrowed Securities		31,800,000	19,357,000
Securities Held as Collateral, at Fair Value		0
Securities Financing Transaction, Fair Value
Securities Financing Transaction [Line Items]
Loaned securities	[1]	34,364,000	27,964,000
Securities Borrowed, Fair Value of Collateral	[2]	3,367,000	9,342,000
Cash Collateral for Borrowed Securities	[3]	31,799,000	19,357,000
Securities Financing Transaction, Fair Value | US Government Corporations and Agencies Securities [Member]
Securities Financing Transaction [Line Items]
Loaned securities	[1]	90,000	0
Securities Financing Transaction, Fair Value | Corporate bonds and notes
Securities Financing Transaction [Line Items]
Loaned securities	[1]	7,427,000	13,862,000
Securities Financing Transaction, Fair Value | Foreign government and agency securities
Securities Financing Transaction [Line Items]
Loaned securities	[1]	0	867,000
Securities Financing Transaction, Fair Value | Equity securities
Securities Financing Transaction [Line Items]
Loaned securities	[1]	26,847,000	13,235,000
Securities Financing Transaction, Cost [Member]
Securities Financing Transaction [Line Items]
Loaned securities		$ 34,651,000	$ 27,846,000
Minimum [Member]
Securities Financing Transaction [Line Items]
Securities Lending Rate of Collateral Required		1.02
Minimum [Member] | Foreign government and agency securities
Securities Financing Transaction [Line Items]
Securities Lending Rate of Collateral Required		1.05
Minimum [Member] | Securities Financing Transaction, Fair Value
Securities Financing Transaction [Line Items]
Securities Lending Rate of Collateral Required		1

[1] Our securities loaned under securities lending agreements are reported at fair value within other assets in our condensed 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 condensed 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 in our condensed consolidated balance sheets. Amounts payable on the return of cash collateral under securities lending agreements are included within other liabilities in our condensed consolidated balance sheets.