Note 5 - Fair Value of Financial Instruments Fair Value Assets Liabilities by Hierarchy Level (Details) - USD ($)	12 Months Ended
	Dec. 31, 2019		Dec. 31, 2018
Fair Value by Hierarchy Level [Line Items]
Loaned securities	$ 66,442,000		$ 27,860,000
Other Investments	4,072,000		3,415,000
Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value	5,654,675,000	[1]	5,149,614,000	[2]
Total Assets at Fair Value	5,721,117,000	[1]	5,177,474,000	[2]
Fixed-maturities available for sale | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value	4,688,911,000		4,021,575,000
Trading securities | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value	317,150,000		469,071,000
Equity securities | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value	130,221,000		130,565,000
Short-term investments | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value	518,393,000		528,403,000
Partnership investment | Carrying (Reported) Amount, Fair Value Disclosure
Fair Value by Hierarchy Level [Line Items]
Other Investments	2,600,000		3,400,000
Convertible promissory note | Carrying (Reported) Amount, Fair Value Disclosure
Fair Value by Hierarchy Level [Line Items]
Other Investments	1,500,000
Fair Value, Inputs, Level 1 | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value	597,506,000	[1]	411,054,000	[2]
Total Assets at Fair Value	660,279,000	[1]	431,096,000	[2]
Fair Value, Inputs, Level 1 | Fixed-maturities available for sale | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value	143,884,000		55,658,000
Fair Value, Inputs, Level 1 | Trading securities | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value	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	124,009,000		126,607,000
Fair Value, Inputs, Level 1 | Short-term investments | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value	329,613,000		228,789,000
Fair Value, Inputs, Level 2 | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value	5,057,169,000	[1]	4,738,560,000	[2]
Total Assets at Fair Value	5,060,838,000	[1]	4,746,378,000	[2]
Fair Value, Inputs, Level 2 | Fixed-maturities available for sale | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value	4,545,027,000		3,965,917,000
Fair Value, Inputs, Level 2 | Trading securities | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value	317,150,000		469,071,000
Fair Value, Inputs, Level 2 | Equity securities | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value	6,212,000		3,958,000
Fair Value, Inputs, Level 2 | Short-term investments | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value	188,780,000		299,614,000
Fair Value, Inputs, Level 3 | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total Assets at Fair Value	0		0
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
Financial Liabilities Fair Value Disclosure	0		0
Securities Financing Transaction, Fair Value | Fixed-maturities available for sale
Fair Value by Hierarchy Level [Line Items]
Loaned securities	24,000,000.0
Securities Financing Transaction, Fair Value | Equity securities
Fair Value by Hierarchy Level [Line Items]
Loaned securities	27,500,000
Securities Financing Transaction, Fair Value | Short-term investments
Fair Value by Hierarchy Level [Line Items]
Loaned securities	14,900,000
US government and agency securities | Fixed-maturities available for sale | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value	179,584,000		84,070,000
US government and agency securities | Short-term investments | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value	127,152,000		133,657,000
US government and agency securities | Fair Value, Inputs, Level 1 | Fixed-maturities available for sale | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value	143,884,000		55,658,000
US government and agency securities | Fair Value, Inputs, Level 1 | Short-term investments | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value	127,152,000		133,657,000
US government and agency securities | Fair Value, Inputs, Level 2 | Fixed-maturities available for sale | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value	35,700,000		28,412,000
US government and agency securities | Fair Value, Inputs, Level 2 | Short-term investments | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value	0		0
US government and agency securities | Securities Financing Transaction, Fair Value | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Loaned securities	35,309,000	[3]	9,987,000	[4]
US government and agency securities | Securities Financing Transaction, Fair Value | Fair Value, Inputs, Level 1 | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Loaned securities	35,309,000	[3]	9,987,000	[4]
US government and agency securities | Securities Financing Transaction, Fair Value | Fair Value, Inputs, Level 2 | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Loaned securities	0	[3]	0	[4]
State and municipal obligations | Fixed-maturities available for sale | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value	119,994,000		138,313,000
State and municipal obligations | Trading securities | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value	118,949,000		168,359,000
State and municipal obligations | Short-term investments | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value	21,475,000		18,070,000
State and municipal obligations | Fair Value, Inputs, Level 1 | Fixed-maturities available for sale | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value	0		0
State and municipal obligations | Fair Value, Inputs, Level 1 | Trading securities | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value	0		0
State and municipal obligations | Fair Value, Inputs, Level 1 | Short-term investments | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value	0		0
State and municipal obligations | Fair Value, Inputs, Level 2 | Fixed-maturities available for sale | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value	119,994,000		138,313,000
State and municipal obligations | Fair Value, Inputs, Level 2 | Trading securities | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value	118,949,000		168,359,000
State and municipal obligations | Fair Value, Inputs, Level 2 | Short-term investments | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value	21,475,000		18,070,000
Money market instruments | Short-term investments | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value	202,461,000		95,132,000
Money market instruments | Fair Value, Inputs, Level 1 | Short-term investments | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value	202,461,000		95,132,000
Money market instruments | Fair Value, Inputs, Level 2 | Short-term investments | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value	0		0
Corporate bonds and notes | Fixed-maturities available for sale | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value	2,237,611,000		2,222,473,000
Corporate bonds and notes | Trading securities | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value	147,232,000		228,152,000
Corporate bonds and notes | Short-term investments | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value	20,298,000		105,625,000
Corporate bonds and notes | Fair Value, Inputs, Level 1 | Fixed-maturities available for sale | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value	0		0
Corporate bonds and notes | Fair Value, Inputs, Level 1 | Trading securities | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value	0		0
Corporate bonds and notes | Fair Value, Inputs, Level 1 | Short-term investments | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value	0		0
Corporate bonds and notes | Fair Value, Inputs, Level 2 | Fixed-maturities available for sale | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value	2,237,611,000		2,222,473,000
Corporate bonds and notes | Fair Value, Inputs, Level 2 | Trading securities | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value	147,232,000		228,152,000
Corporate bonds and notes | Fair Value, Inputs, Level 2 | Short-term investments | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value	20,298,000		105,625,000
Corporate bonds and notes | Securities Financing Transaction, Fair Value | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Loaned securities	3,669,000	[3]	7,818,000	[4]
Corporate bonds and notes | Securities Financing Transaction, Fair Value | Fair Value, Inputs, Level 1 | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Loaned securities	0	[3]	0	[4]
Corporate bonds and notes | Securities Financing Transaction, Fair Value | Fair Value, Inputs, Level 2 | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Loaned securities	3,669,000	[3]	7,818,000	[4]
RMBS | Fixed-maturities available for sale | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value	779,354,000		332,142,000
RMBS | Trading securities | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value	16,180,000		21,082,000
RMBS | Fair Value, Inputs, Level 1 | Fixed-maturities available for sale | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value	0		0
RMBS | Fair Value, Inputs, Level 1 | Trading securities | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value	0		0
RMBS | Fair Value, Inputs, Level 2 | Fixed-maturities available for sale | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value	779,354,000		332,142,000
RMBS | Fair Value, Inputs, Level 2 | Trading securities | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value	16,180,000		21,082,000
CMBS | Fixed-maturities available for sale | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value	608,015,000		539,915,000
CMBS | Trading securities | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value	34,789,000		51,478,000
CMBS | Fair Value, Inputs, Level 1 | Fixed-maturities available for sale | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value	0		0
CMBS | Fair Value, Inputs, Level 1 | Trading securities | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value	0		0
CMBS | Fair Value, Inputs, Level 2 | Fixed-maturities available for sale | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value	608,015,000		539,915,000
CMBS | Fair Value, Inputs, Level 2 | Trading securities | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value	34,789,000		51,478,000
Other ABS | Fixed-maturities available for sale | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value	759,129,000		704,662,000
Other ABS | Short-term investments | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value			806,000
Other ABS | Fair Value, Inputs, Level 1 | Fixed-maturities available for sale | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value	0		0
Other ABS | Fair Value, Inputs, Level 1 | Short-term investments | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value			0
Other ABS | Fair Value, Inputs, Level 2 | Fixed-maturities available for sale | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value	759,129,000		704,662,000
Other ABS | Fair Value, Inputs, Level 2 | Short-term investments | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value			806,000
Foreign government and agency securities | Fixed-maturities available for sale | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value	5,224,000
Foreign government and agency securities | Fair Value, Inputs, Level 1 | Fixed-maturities available for sale | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value	0
Foreign government and agency securities | Fair Value, Inputs, Level 2 | Fixed-maturities available for sale | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value	5,224,000
Other investments | Short-term investments | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value	147,007,000	[5]	175,113,000	[6]
Other investments | Fair Value, Inputs, Level 1 | Short-term investments | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value	0	[5]	0	[6]
Other investments | Fair Value, Inputs, Level 2 | Short-term investments | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value	147,007,000	[5]	175,113,000	[6]
Equity securities | Securities Financing Transaction, Fair Value | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Loaned securities	27,464,000	[3]	10,055,000	[4]
Equity securities | Securities Financing Transaction, Fair Value | Fair Value, Inputs, Level 1 | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Loaned securities	27,464,000	[3]	10,055,000	[4]
Equity securities | Securities Financing Transaction, Fair Value | Fair Value, Inputs, Level 2 | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Loaned securities	$ 0	[3]	$ 0	[4]

[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 $3.4 million that are primarily invested in limited partnerships valued using the net asset value as a practical expedient.

[3] Securities loaned to third-party borrowers under securities lending agreements are classified as other assets in our consolidated balance sheets. See Note 6 for more information.

[4] Securities loaned to third-party borrowers under securities lending agreements are classified as other assets in our consolidated balance sheets. See Note 6 for more information.

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

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