Note 4 - Fair Value of Financial Instruments Assets Measured at Fair Value by Hierarchy Level (Details) - USD ($)		3 Months Ended		12 Months Ended
		Mar. 31, 2020		Dec. 31, 2019
Fair Value by Hierarchy Level [Line Items]
Other Investments		$ 4,131,000		$ 4,072,000
Loaned securities		47,520,000		66,442,000
Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value		5,604,496,000	[1]	5,654,675,000	[2]
Assets, Fair Value Disclosure		5,652,016,000	[1]	5,721,117,000	[2]
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Transfers, Net		0		0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Transfers, Net		0		0
Fair Value, Measurements, Recurring | Investments Measured at NAV
Fair Value by Hierarchy Level [Line Items]
Other Investments		2,600,000
Fair Value, Inputs, Level 1 | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value		659,008,000	[1]	597,506,000	[2]
Assets, Fair Value Disclosure		702,744,000	[1]	660,279,000	[2]
Fair Value, Inputs, Level 2 | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value		4,945,488,000	[1]	5,057,169,000	[2]
Assets, Fair Value Disclosure		4,949,272,000	[1]	5,060,838,000	[2]
Fair Value, Inputs, Level 3 | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Assets, Fair Value Disclosure		0		400,000
Financial Liabilities Fair Value Disclosure		5,600,000		0
Securities Financing Transaction, Fair Value | Fair Value, Measurements, Recurring | US government and agency securities
Fair Value by Hierarchy Level [Line Items]
Loaned securities		3,784,000	[3]	35,309,000	[4]
Securities Financing Transaction, Fair Value | Fair Value, Measurements, Recurring | Corporate bonds and notes
Fair Value by Hierarchy Level [Line Items]
Loaned securities	[4]			3,669,000
Securities Financing Transaction, Fair Value | Fair Value, Measurements, Recurring | Equity securities
Fair Value by Hierarchy Level [Line Items]
Loaned securities		43,736,000	[3]	27,464,000	[4]
Securities Financing Transaction, Fair Value | Fair Value, Inputs, Level 1 | Fair Value, Measurements, Recurring | US government and agency securities
Fair Value by Hierarchy Level [Line Items]
Loaned securities		0	[3]	35,309,000	[4]
Securities Financing Transaction, Fair Value | Fair Value, Inputs, Level 1 | Fair Value, Measurements, Recurring | Corporate bonds and notes
Fair Value by Hierarchy Level [Line Items]
Loaned securities	[4]			0
Securities Financing Transaction, Fair Value | Fair Value, Inputs, Level 1 | Fair Value, Measurements, Recurring | Equity securities
Fair Value by Hierarchy Level [Line Items]
Loaned securities		43,736,000	[3]	27,464,000	[4]
Securities Financing Transaction, Fair Value | Fair Value, Inputs, Level 2 | Fair Value, Measurements, Recurring | US government and agency securities
Fair Value by Hierarchy Level [Line Items]
Loaned securities		3,784,000	[3]	0	[4]
Securities Financing Transaction, Fair Value | Fair Value, Inputs, Level 2 | Fair Value, Measurements, Recurring | Corporate bonds and notes
Fair Value by Hierarchy Level [Line Items]
Loaned securities	[4]			3,669,000
Securities Financing Transaction, Fair Value | Fair Value, Inputs, Level 2 | Fair Value, Measurements, Recurring | Equity securities
Fair Value by Hierarchy Level [Line Items]
Loaned securities		0	[3]	0	[4]
Fixed-maturities available for sale | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value		4,603,613,000		4,688,911,000
Fixed-maturities available for sale | Fair Value, Measurements, Recurring | US government and agency securities
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value		159,240,000		179,584,000
Fixed-maturities available for sale | Fair Value, Measurements, Recurring | State and municipal obligations
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value		116,975,000		119,994,000
Fixed-maturities available for sale | Fair Value, Measurements, Recurring | Corporate bonds and notes
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value		2,265,066,000		2,237,611,000
Fixed-maturities available for sale | Fair Value, Measurements, Recurring | RMBS
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value		779,590,000		779,354,000
Fixed-maturities available for sale | Fair Value, Measurements, Recurring | CMBS
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value		610,877,000		608,015,000
Fixed-maturities available for sale | Fair Value, Measurements, Recurring | Other ABS
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value		666,918,000		759,129,000
Fixed-maturities available for sale | Fair Value, Measurements, Recurring | Foreign government and agency securities
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value		4,947,000		5,224,000
Fixed-maturities available for sale | Fair Value, Inputs, Level 1 | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value		155,498,000		143,884,000
Fixed-maturities available for sale | 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		155,498,000		143,884,000
Fixed-maturities available for sale | 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
Fixed-maturities available for sale | 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
Fixed-maturities available for sale | Fair Value, Inputs, Level 1 | Fair Value, Measurements, Recurring | RMBS
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value		0		0
Fixed-maturities available for sale | Fair Value, Inputs, Level 1 | Fair Value, Measurements, Recurring | CMBS
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value		0		0
Fixed-maturities available for sale | 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
Fixed-maturities available for sale | Fair Value, Inputs, Level 1 | Fair Value, Measurements, Recurring | Foreign government and agency securities
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value		0		0
Fixed-maturities available for sale | Fair Value, Inputs, Level 2 | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value		4,448,115,000		4,545,027,000
Fixed-maturities available for sale | 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		3,742,000		35,700,000
Fixed-maturities available for sale | 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		116,975,000		119,994,000
Fixed-maturities available for sale | 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		2,265,066,000		2,237,611,000
Fixed-maturities available for sale | Fair Value, Inputs, Level 2 | Fair Value, Measurements, Recurring | RMBS
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value		779,590,000		779,354,000
Fixed-maturities available for sale | Fair Value, Inputs, Level 2 | Fair Value, Measurements, Recurring | CMBS
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value		610,877,000		608,015,000
Fixed-maturities available for sale | Fair Value, Inputs, Level 2 | Fair Value, Measurements, Recurring | Other ABS
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value		666,918,000		759,129,000
Fixed-maturities available for sale | Fair Value, Inputs, Level 2 | Fair Value, Measurements, Recurring | Foreign government and agency securities
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value		4,947,000		5,224,000
Trading Securities | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value		287,270,000		317,150,000
Trading Securities | Fair Value, Measurements, Recurring | State and municipal obligations
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value		114,511,000		118,949,000
Trading Securities | Fair Value, Measurements, Recurring | Corporate bonds and notes
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value		123,461,000		147,232,000
Trading Securities | Fair Value, Measurements, Recurring | RMBS
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value		15,573,000		16,180,000
Trading Securities | Fair Value, Measurements, Recurring | CMBS
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value		33,725,000		34,789,000
Trading Securities | Fair Value, Inputs, Level 1 | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value		0		0
Trading Securities | 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
Trading Securities | 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
Trading Securities | Fair Value, Inputs, Level 1 | Fair Value, Measurements, Recurring | RMBS
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value		0		0
Trading Securities | Fair Value, Inputs, Level 1 | Fair Value, Measurements, Recurring | CMBS
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value		0		0
Trading Securities | Fair Value, Inputs, Level 2 | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value		287,270,000		317,150,000
Trading Securities | 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		114,511,000		118,949,000
Trading Securities | 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		123,461,000		147,232,000
Trading Securities | Fair Value, Inputs, Level 2 | Fair Value, Measurements, Recurring | RMBS
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value		15,573,000		16,180,000
Trading Securities | Fair Value, Inputs, Level 2 | Fair Value, Measurements, Recurring | CMBS
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value		33,725,000		34,789,000
Equity securities | Fair Value, Measurements, Recurring | Equity securities
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value		74,853,000		130,221,000
Equity securities | Fair Value, Inputs, Level 1 | Fair Value, Measurements, Recurring | Equity securities
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value		67,086,000		124,009,000
Equity securities | Fair Value, Inputs, Level 2 | Fair Value, Measurements, Recurring | Equity securities
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value		7,767,000		6,212,000
Short-term investments | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value		638,760,000		518,393,000
Short-term investments | Fair Value, Measurements, Recurring | US government and agency securities
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value		140,667,000		127,152,000
Short-term investments | Fair Value, Measurements, Recurring | State and municipal obligations
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value		10,148,000		21,475,000
Short-term investments | Fair Value, Measurements, Recurring | Money market instruments
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value		295,757,000		202,461,000
Short-term investments | Fair Value, Measurements, Recurring | Corporate bonds and notes
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value		117,836,000		20,298,000
Short-term investments | Fair Value, Measurements, Recurring | Other investments
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value		74,352,000	[5]	147,007,000	[6]
Short-term investments | Fair Value, Inputs, Level 1 | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value		436,424,000		329,613,000
Short-term investments | 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		140,667,000		127,152,000
Short-term investments | 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
Short-term investments | Fair Value, Inputs, Level 1 | Fair Value, Measurements, Recurring | Money market instruments
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value		295,757,000		202,461,000
Short-term investments | 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
Short-term investments | Fair Value, Inputs, Level 1 | Fair Value, Measurements, Recurring | Other investments
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value		0	[5]	0	[6]
Short-term investments | Fair Value, Inputs, Level 2 | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value		202,336,000		188,780,000
Short-term investments | 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		0		0
Short-term investments | 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		10,148,000		21,475,000
Short-term investments | 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
Short-term investments | 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		117,836,000		20,298,000
Short-term investments | Fair Value, Inputs, Level 2 | Fair Value, Measurements, Recurring | Other investments
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value		74,352,000	[5]	147,007,000	[6]
Carrying (Reported) Amount, Fair Value Disclosure | Partnership investment
Fair Value by Hierarchy Level [Line Items]
Other Investments				2,600,000
Carrying (Reported) Amount, Fair Value Disclosure | Convertible promissory note
Fair Value by Hierarchy Level [Line Items]
Other Investments		$ 1,500,000		$ 1,500,000

[1] Does not include other invested assets of $2.6 million that are primarily invested in limited partnership investments valued using the net asset value as a practical expedient and $1.5 million invested in a private convertible promissory note.

[2] Does not include other invested assets of $2.6 million that are primarily invested in limited partnership investments valued using the net asset value as a practical expedient and $1.5 million invested in a private convertible promissory note.

[3] Securities loaned to third-party borrowers under securities lending agreements are classified as other assets in our condensed consolidated balance sheets. See Note 5 for more information.

[4] Securities loaned to third-party borrowers under securities lending agreements are classified as other assets in our condensed consolidated balance sheets. See Note 5 for more information.

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

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