Note 4 - Fair Value of Financial Instruments Fair Value Assets Liabilities by Hierarchy Level (Details) - USD ($)		3 Months Ended			6 Months Ended
		Jun. 30, 2018		Jun. 30, 2017	Jun. 30, 2018		Jun. 30, 2017	Dec. 31, 2017
Fair Value by Hierarchy Level [Line Items]
Other Investments		$ 3,193,000			$ 3,193,000			$ 334,000
Securities Received as Collateral		39,224,000			39,224,000			19,357,000
Loaned securities		38,350,000			38,350,000			27,964,000
Gain (Loss) on Investments		(6,821,000)		$ 4,756,000	(23,911,000)		$ 1,803,000
Other than Temporary Impairment Losses, Investments		0		1,000,000	844,000		1,000,000
Carrying (Reported) Amount, Fair Value Disclosure [Member]
Fair Value by Hierarchy Level [Line Items]
Other Investments		3,200,000			3,200,000
Cost Method Investments	[1]	0			0			334,000
Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value		4,909,077,000	[2],[3]		4,909,077,000	[2],[3]		4,671,572,000	[4],[5]
Total Assets at Fair Value		4,909,077,000	[3]		4,909,077,000	[3]		4,671,572,000	[5]
Fair Value, Assets, Level 2 to Level 1 Transfers, Amount		0		0	0		0
Fair Value, Assets, Level 1 to Level 2 Transfers, Amount		0		0	0		0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Transfers, Net		0		0	0		0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Transfers, Net		0		$ 0	0		$ 0
US government and agency securities | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value		157,206,000			157,206,000			132,992,000
State and municipal obligations | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value		351,085,000			351,085,000			386,111,000
Money market instruments | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value		211,348,000			211,348,000			213,357,000
Corporate bonds and notes | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value		2,331,887,000			2,331,887,000			2,304,017,000
RMBS | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value		268,449,000			268,449,000			216,749,000
CMBS | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value		527,098,000			527,098,000			503,955,000
Other ABS | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value		693,672,000			693,672,000			676,158,000
Foreign government securities | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value		50,433,000			50,433,000			36,448,000
Equity securities | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value		144,906,000			144,906,000			176,065,000
Other investments | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value		172,993,000	[6]		172,993,000	[6]		25,720,000	[7]
Fair Value, Inputs, Level 1 | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value		488,734,000	[2]		488,734,000	[2]		513,531,000	[4]
Total Assets at Fair Value		488,734,000			488,734,000			513,531,000
Fair Value, Inputs, Level 1 | US government and agency securities | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value		137,749,000			137,749,000			124,969,000
Fair Value, Inputs, Level 1 | State and municipal obligations | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value		0			0			0
Fair Value, Inputs, Level 1 | Money market instruments | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value		211,348,000			211,348,000			213,357,000
Fair Value, Inputs, Level 1 | Corporate bonds and notes | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value		0			0			0
Fair Value, Inputs, Level 1 | RMBS | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value		0			0			0
Fair Value, Inputs, Level 1 | CMBS | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value		0			0			0
Fair Value, Inputs, Level 1 | Other ABS | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value		0			0			0
Fair Value, Inputs, Level 1 | Foreign government securities | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value		0			0			0
Fair Value, Inputs, Level 1 | Equity securities | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value		139,637,000			139,637,000			175,205,000
Fair Value, Inputs, Level 1 | Other investments | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value		0	[6]		0	[6]		0	[7]
Fair Value, Inputs, Level 2 | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value		4,420,343,000	[2]		4,420,343,000	[2]		4,158,041,000	[4]
Total Assets at Fair Value		4,420,343,000			4,420,343,000			4,158,041,000
Fair Value, Inputs, Level 2 | US government and agency securities | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value		19,457,000			19,457,000			8,023,000
Fair Value, Inputs, Level 2 | State and municipal obligations | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value		351,085,000			351,085,000			386,111,000
Fair Value, Inputs, Level 2 | Money market instruments | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value		0			0			0
Fair Value, Inputs, Level 2 | Corporate bonds and notes | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value		2,331,887,000			2,331,887,000			2,304,017,000
Fair Value, Inputs, Level 2 | RMBS | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value		268,449,000			268,449,000			216,749,000
Fair Value, Inputs, Level 2 | CMBS | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value		527,098,000			527,098,000			503,955,000
Fair Value, Inputs, Level 2 | Other ABS | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value		693,672,000			693,672,000			676,158,000
Fair Value, Inputs, Level 2 | Foreign government securities | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value		50,433,000			50,433,000			36,448,000
Fair Value, Inputs, Level 2 | Equity securities | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value		5,269,000			5,269,000			860,000
Fair Value, Inputs, Level 2 | Other investments | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value		172,993,000	[6]		172,993,000	[6]		25,720,000	[7]
Fair Value, Inputs, Level 3 | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total Assets at Fair Value		0			0			0
Fair Value, Measurement with Unobservable Inputs Reconciliations, Recurring Basis, Liability Value		0			0			0
Securities Financing Transaction, Fair Value [Member]
Fair Value by Hierarchy Level [Line Items]
Securities Received as Collateral	[8],[9]	39,224,000			39,224,000			19,357,000
Loaned securities	[9]	$ 38,350,000			$ 38,350,000			$ 27,964,000

[1] As a result of implementing the update to the standard for the accounting of financial instruments effective January 1, 2018, other invested assets are no longer carried at amortized cost.

[2] Does not include certain other invested assets ($3.2 million), primarily invested in limited partnership investments valued using the net asset value as a practical expedient. Includes cash collateral held under securities lending agreements ($39.2 million) reinvested in money market instruments.

[3] Includes $38.4 million of securities loaned to third-party Borrowers under securities lending agreements, classified as other assets in our consolidated balance sheets. See Note 5 for more information.

[4] Does not include certain other invested assets ($0.3 million), primarily invested in limited partnerships, accounted for as cost-method investments and not measured at fair value. Includes cash collateral held under securities lending agreements ($19.4 million) reinvested in money market instruments.

[5] Includes $28.0 million of securities loaned to third-party Borrowers under securities lending agreements, classified as other assets in our consolidated balance sheets. See Note 5 for more information.

[6] Comprising short-term certificates of deposit and commercial paper.

[7] Comprising short-term certificates of deposit and commercial paper.

[8] 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.

[9] 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.