Note 4 - Fair Value of Financial Instruments Assets Measured at Fair Value by Hierarchy Level (Details) - USD ($)		3 Months Ended		6 Months Ended		12 Months Ended
		Jun. 30, 2020		Jun. 30, 2020		Dec. 31, 2019
Fair Value by Hierarchy Level [Line Items]
Other Investments		$ 4,047,000		$ 4,047,000		$ 4,072,000
Loaned securities		16,332,000		16,332,000		66,442,000
Investments Measured at NAV
Fair Value by Hierarchy Level [Line Items]
Other Investments		2,500,000		2,500,000
Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value		6,427,303,000	[1]	6,427,303,000	[1]	5,654,675,000	[2]
Assets, Fair Value Disclosure		6,443,635,000	[1]	6,443,635,000	[1]	5,721,117,000	[2]
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Transfers, Net		0		0		0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Transfers, Net		0		0		0
Fair Value, Inputs, Level 1 | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value		638,313,000	[1]	638,313,000	[1]	597,506,000	[2]
Assets, Fair Value Disclosure		652,526,000	[1]	652,526,000	[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		5,788,990,000	[1]	5,788,990,000	[1]	5,057,169,000	[2]
Assets, Fair Value Disclosure		5,791,109,000	[1]	5,791,109,000	[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		7,600,000		7,600,000		400,000
Securities Financing Transaction, Fair Value | Fair Value, Measurements, Recurring | US government and agency securities
Fair Value by Hierarchy Level [Line Items]
Loaned securities	[3]					35,309,000
Securities Financing Transaction, Fair Value | Fair Value, Measurements, Recurring | Corporate bonds and notes
Fair Value by Hierarchy Level [Line Items]
Loaned securities		2,119,000	[4]	2,119,000	[4]	3,669,000	[3]
Securities Financing Transaction, Fair Value | Fair Value, Measurements, Recurring | Equity securities
Fair Value by Hierarchy Level [Line Items]
Loaned securities		14,213,000	[4]	14,213,000	[4]	27,464,000	[3]
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	[3]					35,309,000
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		0	[4]	0	[4]	0	[3]
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		14,213,000	[4]	14,213,000	[4]	27,464,000	[3]
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]					0
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		2,119,000	[4]	2,119,000	[4]	3,669,000	[3]
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	[4]	0	[4]	0	[3]
Fixed-maturities available for sale | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value		5,070,091,000		5,070,091,000		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		133,043,000		133,043,000		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		123,117,000		123,117,000		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		2,648,456,000		2,648,456,000		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		835,769,000		835,769,000		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		637,491,000		637,491,000		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		686,875,000		686,875,000		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		5,340,000		5,340,000		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		124,600,000		124,600,000		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		124,600,000		124,600,000		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		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		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		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		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		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		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		4,945,491,000		4,945,491,000		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		8,443,000		8,443,000		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		123,117,000		123,117,000		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		2,648,456,000		2,648,456,000		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		835,769,000		835,769,000		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		637,491,000		637,491,000		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		686,875,000		686,875,000		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		5,340,000		5,340,000		5,224,000
Fixed-maturities available for sale | Securities Financing Transaction, Fair Value | Corporate bonds and notes
Fair Value by Hierarchy Level [Line Items]
Loaned securities						24,013,000
Trading Securities | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value		297,609,000		297,609,000		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		122,234,000		122,234,000		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		126,000,000		126,000,000		147,232,000
Trading Securities | Fair Value, Measurements, Recurring | RMBS
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value		15,205,000		15,205,000		16,180,000
Trading Securities | Fair Value, Measurements, Recurring | CMBS
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value		34,170,000		34,170,000		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		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		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		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		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		0
Trading Securities | Fair Value, Inputs, Level 2 | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value		297,609,000		297,609,000		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		122,234,000		122,234,000		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		126,000,000		126,000,000		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		15,205,000		15,205,000		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		34,170,000		34,170,000		34,789,000
Equity securities | Fair Value, Measurements, Recurring | Equity securities
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value		104,513,000		104,513,000		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		96,089,000		96,089,000		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		8,424,000		8,424,000		6,212,000
Short-term investments | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value		955,090,000		955,090,000		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		42,528,000		42,528,000		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		5,397,000		5,397,000		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		375,096,000		375,096,000		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		149,490,000		149,490,000		20,298,000
Short-term investments | Fair Value, Measurements, Recurring | Other investments
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value		382,579,000	[5]	382,579,000	[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		417,624,000		417,624,000		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		42,528,000		42,528,000		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		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		375,096,000		375,096,000		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		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	[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		537,466,000		537,466,000		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		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		5,397,000		5,397,000		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		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		149,490,000		149,490,000		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		382,579,000	[5]	382,579,000	[5]	147,007,000	[6]
Partnership investment
Fair Value by Hierarchy Level [Line Items]
Other Investments						2,600,000
Convertible promissory note
Fair Value by Hierarchy Level [Line Items]
Other Investments		$ 1,500,000		$ 1,500,000		$ 1,500,000

[1] Does not include other invested assets of $2.5 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.