Note 5 - Fair Value of Financial Instruments - Fair Value Assets Liabilities by Hierarchy Level (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Fair Value by Hierarchy Level [Line Items]
Loaned securities	$ 57,499	$ 66,442
Other Investments	4,973	4,072
Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value	6,786,469	5,656,175
Derivative, Fair Value, Net	5,513	383
Total Assets at Fair Value	6,849,481	5,723,000
Fixed-maturities available for sale | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value	5,723,340	4,688,911
Trading securities | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value	290,885	317,150
Equity securities | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value	151,240	130,221
Short-term investments | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value	618,004	518,393
Partnership investment | Carrying (Reported) Amount, Fair Value Disclosure
Fair Value by Hierarchy Level [Line Items]
Other Investments	2,000	2,600
Other investments | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value	3,000	1,500
Fair Value, Inputs, Level 1 | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value	551,695	597,506
Derivative, Fair Value, Net	0	0
Total Assets at Fair Value	577,870	660,279
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	140,034	143,884
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	142,761	124,009
Fair Value, Inputs, Level 1 | Short-term investments | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value	268,900	329,613
Fair Value, Inputs, Level 1 | Other investments | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value	0	0
Fair Value, Inputs, Level 2 | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value	6,231,774	5,057,169
Derivative, Fair Value, Net	0	0
Total Assets at Fair Value	6,263,098	5,060,838
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	5,583,306	4,545,027
Fair Value, Inputs, Level 2 | Trading securities | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value	290,885	317,150
Fair Value, Inputs, Level 2 | Equity securities | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value	8,479	6,212
Fair Value, Inputs, Level 2 | Short-term investments | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value	349,104	188,780
Fair Value, Inputs, Level 2 | Other investments | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value	0	0
Fair Value, Inputs, Level 3 | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value	3,000	1,500
Derivative, Fair Value, Net	5,513	383
Total Assets at Fair Value	8,513	1,883
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, Inputs, Level 3 | Fixed-maturities available for sale | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value	0	0
Fair Value, Inputs, Level 3 | Trading securities | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value	0	0
Fair Value, Inputs, Level 3 | Equity securities | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value	0	0
Fair Value, Inputs, Level 3 | Short-term investments | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value	0	0
Fair Value, Inputs, Level 3 | Other investments | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value	3,000	1,500
Securities Financing Transaction, Fair Value | Fixed-maturities available for sale
Fair Value by Hierarchy Level [Line Items]
Loaned securities	36,100
Securities Financing Transaction, Fair Value | Equity securities
Fair Value by Hierarchy Level [Line Items]
Loaned securities	21,300
Securities Financing Transaction, Fair Value | Short-term investments
Fair Value by Hierarchy Level [Line Items]
Loaned securities	100
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	169,223	179,584
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
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	140,034	143,884
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
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	29,189	35,700
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
US government and agency securities | Fair Value, Inputs, Level 3 | Fixed-maturities available for sale | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value	0	0
US government and agency securities | Fair Value, Inputs, Level 3 | Short-term investments | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value		0
US government and agency securities | Securities Financing Transaction, Fair Value | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Loaned securities	4,876	35,309
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	4,876	35,309
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	0
US government and agency securities | Securities Financing Transaction, Fair Value | Fair Value, Inputs, Level 3 | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Loaned securities	0	0
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	165,271	119,994
State and municipal obligations | Trading securities | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value	120,449	118,949
State and municipal obligations | Short-term investments | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value	21,819	21,475
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	165,271	119,994
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	120,449	118,949
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	21,819	21,475
State and municipal obligations | Fair Value, Inputs, Level 3 | 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 3 | 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 3 | Short-term investments | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value	0	0
Money market instruments | Short-term investments | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value	268,900	202,461
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	268,900	202,461
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
Money market instruments | Fair Value, Inputs, Level 3 | 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	3,047,189	2,237,611
Corporate bonds and notes | Trading securities | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value	123,142	147,232
Corporate bonds and notes | Short-term investments | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value	30,495	20,298
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	3,047,189	2,237,611
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	123,142	147,232
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	30,495	20,298
Corporate bonds and notes | Fair Value, Inputs, Level 3 | 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 3 | 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 3 | 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 | Securities Financing Transaction, Fair Value | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Loaned securities	31,324	3,669
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	0
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	31,324	3,669
Corporate bonds and notes | Securities Financing Transaction, Fair Value | Fair Value, Inputs, Level 3 | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Loaned securities	0	0
RMBS | Fixed-maturities available for sale | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value	833,939	779,354
RMBS | Trading securities | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value	13,000	16,180
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	833,939	779,354
RMBS | Fair Value, Inputs, Level 2 | Trading securities | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value	13,000	16,180
RMBS | Fair Value, Inputs, Level 3 | 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 3 | Trading securities | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value	0	0
CMBS | Fixed-maturities available for sale | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value	681,265	588,420
CMBS | Trading securities | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value	34,294	34,789
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	681,265	588,420
CMBS | Fair Value, Inputs, Level 2 | Trading securities | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value	34,294	34,789
CMBS | Fair Value, Inputs, Level 3 | 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 3 | Trading securities | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value	0	0
CLO | Fixed-maturities available for sale | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value	568,558	462,561
CLO | 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
CLO | 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	568,558	462,561
CLO | Fair Value, Inputs, Level 3 | 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 | Fixed-maturities available for sale | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value	252,457	316,163
Other ABS | Short-term investments | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value	219
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	252,457	316,163
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	219
Other ABS | Fair Value, Inputs, Level 3 | 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 3 | Short-term investments | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value	0
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	5,438	5,224
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	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	5,438	5,224
Foreign government and agency securities | Fair Value, Inputs, Level 3 | Fixed-maturities available for sale | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value	0	0
Other investments | Short-term investments | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value	296,571	147,007
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	0
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	296,571	147,007
Other investments | Fair Value, Inputs, Level 3 | Short-term investments | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value	0	0
Equity securities | Securities Financing Transaction, Fair Value | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Loaned securities	21,299	27,464
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	21,299	27,464
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	0
Equity securities | Securities Financing Transaction, Fair Value | Fair Value, Inputs, Level 3 | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Loaned securities	$ 0	$ 0