Note 4 - Fair Value of Financial Instruments Assets Measured at Fair Value by Hierarchy Level (Details) - USD ($)		3 Months Ended			9 Months Ended
		Sep. 30, 2018		Sep. 30, 2017	Sep. 30, 2018		Sep. 30, 2017	Dec. 31, 2017
Fair Value by Hierarchy Level [Line Items]
Other Investments		$ 3,585,000			$ 3,585,000			$ 334,000
Reinvested cash collateral held under securities lending agreements		31,800,000			31,800,000			19,357,000
Loaned securities		34,364,000			34,364,000			27,964,000
Carrying (Reported) Amount, Fair Value Disclosure
Fair Value by Hierarchy Level [Line Items]
Other Investments		3,600,000			3,600,000
Cost Method Investments	[1]	0			0			334,000
Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value		5,059,014,000	[2],[3]		5,059,014,000	[2],[3]		4,671,572,000	[4],[5]
Total Assets at Fair Value		5,059,014,000	[3]		5,059,014,000	[3]		4,671,572,000	[5]
Fair Value, Assets, Level 1 to Level 2 Transfers, Amount		0		$ 0	0		$ 0
Fair Value, Assets, Level 2 to Level 1 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
Fair Value, Measurements, Recurring | US government and agency securities
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value		256,052,000			256,052,000			132,992,000
Fair Value, Measurements, Recurring | State and municipal obligations
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value		345,029,000			345,029,000			386,111,000
Fair Value, Measurements, Recurring | Money market instruments
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value		156,396,000			156,396,000			213,357,000
Fair Value, Measurements, Recurring | Corporate bonds and notes
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value		2,480,097,000			2,480,097,000			2,304,017,000
Fair Value, Measurements, Recurring | RMBS
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value		272,125,000			272,125,000			216,749,000
Fair Value, Measurements, Recurring | CMBS
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value		576,829,000			576,829,000			503,955,000
Fair Value, Measurements, Recurring | Other ABS
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value		674,509,000			674,509,000			676,158,000
Fair Value, Measurements, Recurring | Foreign government securities
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value								36,448,000
Fair Value, Measurements, Recurring | Equity securities
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value		148,712,000			148,712,000			176,065,000
Fair Value, Measurements, Recurring | Other investments
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value		149,265,000	[6]		149,265,000	[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		536,101,000	[2]		536,101,000	[2]		513,531,000	[4]
Total Assets at Fair Value		536,101,000			536,101,000			513,531,000
Fair Value, Inputs, Level 1 | Fair Value, Measurements, Recurring | US government and agency securities
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value		234,507,000			234,507,000			124,969,000
Fair Value, Inputs, Level 1 | Fair Value, Measurements, Recurring | State and municipal obligations
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value		0			0			0
Fair Value, Inputs, Level 1 | Fair Value, Measurements, Recurring | Money market instruments
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value		156,396,000			156,396,000			213,357,000
Fair Value, Inputs, Level 1 | Fair Value, Measurements, Recurring | Corporate bonds and notes
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value		0			0			0
Fair Value, Inputs, Level 1 | Fair Value, Measurements, Recurring | RMBS
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value		0			0			0
Fair Value, Inputs, Level 1 | Fair Value, Measurements, Recurring | CMBS
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value		0			0			0
Fair Value, Inputs, Level 1 | Fair Value, Measurements, Recurring | Other ABS
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value		0			0			0
Fair Value, Inputs, Level 1 | Fair Value, Measurements, Recurring | Foreign government securities
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value								0
Fair Value, Inputs, Level 1 | Fair Value, Measurements, Recurring | Equity securities
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value		145,198,000			145,198,000			175,205,000
Fair Value, Inputs, Level 1 | Fair Value, Measurements, Recurring | Other investments
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		4,522,913,000	[2]		4,522,913,000	[2]		4,158,041,000	[4]
Total Assets at Fair Value		4,522,913,000			4,522,913,000			4,158,041,000
Fair Value, Inputs, Level 2 | Fair Value, Measurements, Recurring | US government and agency securities
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value		21,545,000			21,545,000			8,023,000
Fair Value, Inputs, Level 2 | Fair Value, Measurements, Recurring | State and municipal obligations
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value		345,029,000			345,029,000			386,111,000
Fair Value, Inputs, Level 2 | Fair Value, Measurements, Recurring | Money market instruments
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value		0			0			0
Fair Value, Inputs, Level 2 | Fair Value, Measurements, Recurring | Corporate bonds and notes
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value		2,480,097,000			2,480,097,000			2,304,017,000
Fair Value, Inputs, Level 2 | Fair Value, Measurements, Recurring | RMBS
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value		272,125,000			272,125,000			216,749,000
Fair Value, Inputs, Level 2 | Fair Value, Measurements, Recurring | CMBS
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value		576,829,000			576,829,000			503,955,000
Fair Value, Inputs, Level 2 | Fair Value, Measurements, Recurring | Other ABS
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value		674,509,000			674,509,000			676,158,000
Fair Value, Inputs, Level 2 | Fair Value, Measurements, Recurring | Foreign government securities
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value								36,448,000
Fair Value, Inputs, Level 2 | Fair Value, Measurements, Recurring | Equity securities
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value		3,514,000			3,514,000			860,000
Fair Value, Inputs, Level 2 | Fair Value, Measurements, Recurring | Other investments
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value		149,265,000	[6]		149,265,000	[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 Reconciliation, Recurring Basis, Liability Value		0			0			0
Securities Financing Transaction, Fair Value
Fair Value by Hierarchy Level [Line Items]
Reinvested cash collateral held under securities lending agreements	[8]	31,799,000			31,799,000			19,357,000
Loaned securities	[9]	34,364,000			34,364,000			27,964,000
Securities Financing Transaction, Fair Value | Corporate bonds and notes
Fair Value by Hierarchy Level [Line Items]
Loaned securities	[9]	7,427,000			7,427,000			13,862,000
Securities Financing Transaction, Fair Value | Foreign government securities
Fair Value by Hierarchy Level [Line Items]
Loaned securities	[9]	0			0			867,000
Securities Financing Transaction, Fair Value | Equity securities
Fair Value by Hierarchy Level [Line Items]
Loaned securities	[9]	$ 26,847,000			$ 26,847,000			$ 13,235,000

[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 ($3.6 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 ($31.8 million) reinvested in money market instruments.

[3] Includes $34.4 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.