Note 5 - Fair Value of Financial Instruments - Assets Measured at Fair Value by Hierarchy Level (Details) - USD ($)	3 Months Ended	9 Months Ended	12 Months Ended
	Sep. 30, 2020	Sep. 30, 2020	Dec. 31, 2019
Fair Value by Hierarchy Level [Line Items]
Other Investments	$ 5,472,000	$ 5,472,000	$ 4,072,000
Less: loaned securities	57,922,000	57,922,000	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	6,579,105,000	6,579,105,000	5,654,675,000
Assets, Fair Value Disclosure	6,637,027,000	6,637,027,000	5,721,117,000
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	503,483,000	503,483,000	597,506,000
Assets, Fair Value Disclosure	547,473,000	547,473,000	660,279,000
Fair Value, Inputs, Level 2 | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value	6,075,622,000	6,075,622,000	5,057,169,000
Assets, Fair Value Disclosure	6,089,554,000	6,089,554,000	5,060,838,000
Fair Value, Inputs, Level 3 | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Assets, Fair Value Disclosure	5,500,000	5,500,000	400,000
Securities Financing Transaction, Fair Value | Fair Value, Measurements, Recurring | U.S. government and agencies securities
Fair Value by Hierarchy Level [Line Items]
Less: loaned securities			35,309,000
Securities Financing Transaction, Fair Value | Fair Value, Measurements, Recurring | Corporate bonds and notes
Fair Value by Hierarchy Level [Line Items]
Less: loaned securities	13,932,000	13,932,000	3,669,000
Securities Financing Transaction, Fair Value | Fair Value, Measurements, Recurring | Equity securities
Fair Value by Hierarchy Level [Line Items]
Less: loaned securities	43,990,000	43,990,000	27,464,000
Securities Financing Transaction, Fair Value | Fair Value, Inputs, Level 1 | Fair Value, Measurements, Recurring | U.S. government and agencies securities
Fair Value by Hierarchy Level [Line Items]
Less: loaned securities			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]
Less: loaned securities	0	0	0
Securities Financing Transaction, Fair Value | Fair Value, Inputs, Level 1 | Fair Value, Measurements, Recurring | Equity securities
Fair Value by Hierarchy Level [Line Items]
Less: loaned securities	43,990,000	43,990,000	27,464,000
Securities Financing Transaction, Fair Value | Fair Value, Inputs, Level 2 | Fair Value, Measurements, Recurring | U.S. government and agencies securities
Fair Value by Hierarchy Level [Line Items]
Less: loaned securities			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]
Less: loaned securities	13,932,000	13,932,000	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]
Less: loaned securities	0	0	0
Fixed-maturities available for sale | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value	5,628,992,000	5,628,992,000	4,688,911,000
Fixed-maturities available for sale | Fair Value, Measurements, Recurring | U.S. government and agencies securities
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value	186,901,000	186,901,000	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	159,975,000	159,975,000	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	2,898,357,000	2,898,357,000	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	850,113,000	850,113,000	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	718,962,000	718,962,000	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	809,388,000	809,388,000	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	5,296,000	5,296,000	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	157,407,000	157,407,000	143,884,000
Fixed-maturities available for sale | Fair Value, Inputs, Level 1 | Fair Value, Measurements, Recurring | U.S. government and agencies securities
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value	157,407,000	157,407,000	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	5,471,585,000	5,471,585,000	4,545,027,000
Fixed-maturities available for sale | Fair Value, Inputs, Level 2 | Fair Value, Measurements, Recurring | U.S. government and agencies securities
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value	29,494,000	29,494,000	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	159,975,000	159,975,000	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	2,898,357,000	2,898,357,000	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	850,113,000	850,113,000	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	718,962,000	718,962,000	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	809,388,000	809,388,000	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	5,296,000	5,296,000	5,224,000
Fixed-maturities available for sale | Securities Financing Transaction, Fair Value | Corporate bonds and notes
Fair Value by Hierarchy Level [Line Items]
Less: loaned securities	13,455,000	13,455,000	24,013,000
Trading Securities | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value	291,650,000	291,650,000	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	120,547,000	120,547,000	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	122,858,000	122,858,000	147,232,000
Trading Securities | Fair Value, Measurements, Recurring | RMBS
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value	13,930,000	13,930,000	16,180,000
Trading Securities | Fair Value, Measurements, Recurring | CMBS
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value	34,315,000	34,315,000	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	291,650,000	291,650,000	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	120,547,000	120,547,000	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	122,858,000	122,858,000	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	13,930,000	13,930,000	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	34,315,000	34,315,000	34,789,000
Equity securities | Fair Value, Measurements, Recurring | Equity securities
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value	91,317,000	91,317,000	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	84,255,000	84,255,000	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	7,062,000	7,062,000	6,212,000
Short-term investments | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value	567,146,000	567,146,000	518,393,000
Short-term investments | Fair Value, Measurements, Recurring | U.S. government and agencies securities
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value	29,716,000	29,716,000	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	26,973,000	26,973,000	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	232,105,000	232,105,000	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	106,013,000	106,013,000	20,298,000
Short-term investments | Fair Value, Measurements, Recurring | Other investments
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value	172,339,000	172,339,000	147,007,000
Short-term investments | Fair Value, Inputs, Level 1 | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value	261,821,000	261,821,000	329,613,000
Short-term investments | Fair Value, Inputs, Level 1 | Fair Value, Measurements, Recurring | U.S. government and agencies securities
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value	29,716,000	29,716,000	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	232,105,000	232,105,000	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	0	0
Short-term investments | Fair Value, Inputs, Level 2 | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value	305,325,000	305,325,000	188,780,000
Short-term investments | Fair Value, Inputs, Level 2 | Fair Value, Measurements, Recurring | U.S. government and agencies 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	26,973,000	26,973,000	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	106,013,000	106,013,000	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	172,339,000	172,339,000	147,007,000
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	$ 3,000,000.0	$ 3,000,000.0	$ 1,500,000