Note 6 - Investments - Available for Sale Securities (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Jun. 30, 2020	Dec. 31, 2019
Debt Securities, Available-for-sale [Line Items]
Debt Securities, Available-for-sale, Amortized Cost	$ 5,338,536		$ 4,549,534
Debt Securities, Available-for-sale, Allowance for Credit Loss	2,290
Less: loaned securities	57,922		66,442
Fixed-maturity investments available for sale, Total, Fair Value	5,628,992		4,688,911
Total fixed-maturities available for sale
Debt Securities, Available-for-sale [Line Items]
Available-for-sale Debt Securities, Amortized Cost Basis, Including Loaned Securities	5,351,426
Available-for-sale Securities, Debt Securities, Including Loaned Securities	5,642,447
Securities Financing Transaction, Cost \| Corporate bonds and notes \| Fair Value, Measurements, Recurring
Debt Securities, Available-for-sale [Line Items]
Less: loaned securities	12,890		23,853
Securities Financing Transaction, Fair Value \| U.S. government and agencies securities \| Fair Value, Measurements, Recurring
Debt Securities, Available-for-sale [Line Items]
Less: loaned securities			35,309
Securities Financing Transaction, Fair Value \| Corporate bonds and notes \| Fair Value, Measurements, Recurring
Debt Securities, Available-for-sale [Line Items]
Less: loaned securities	13,932		3,669
Fixed-maturities available for sale \| U.S. government and agencies securities
Debt Securities, Available-for-sale [Line Items]
Available-for-sale Debt Securities, Amortized Cost Basis, Including Loaned Securities	186,457		198,613
Debt Securities, Available-for-sale, Allowance for Credit Loss	0
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax	2,105		2,048
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	(1,661)		(733)
Available-for-sale Securities, Debt Securities, Including Loaned Securities	186,901		199,928
Fixed-maturities available for sale \| State and municipal obligations
Debt Securities, Available-for-sale [Line Items]
Available-for-sale Debt Securities, Amortized Cost Basis, Including Loaned Securities	145,318		112,003
Debt Securities, Available-for-sale, Allowance for Credit Loss	0
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax	14,850		8,032
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	(193)		(41)
Available-for-sale Securities, Debt Securities, Including Loaned Securities	159,975		119,994
Fixed-maturities available for sale \| Corporate bonds and notes
Debt Securities, Available-for-sale [Line Items]
Available-for-sale Debt Securities, Amortized Cost Basis, Including Loaned Securities	2,703,446		2,136,819
Debt Securities, Available-for-sale, Allowance for Credit Loss	(2,290)	$ (2,476)	0
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax	218,127		106,189
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	(7,471)		(1,728)
Available-for-sale Securities, Debt Securities, Including Loaned Securities	2,911,812		2,241,280
Fixed-maturities available for sale \| RMBS
Debt Securities, Available-for-sale [Line Items]
Available-for-sale Debt Securities, Amortized Cost Basis, Including Loaned Securities	813,006		766,429
Debt Securities, Available-for-sale, Allowance for Credit Loss	0
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax	37,137		14,452
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	(30)		(1,527)
Available-for-sale Securities, Debt Securities, Including Loaned Securities	850,113		779,354
Fixed-maturities available for sale \| CMBS
Debt Securities, Available-for-sale [Line Items]
Available-for-sale Debt Securities, Amortized Cost Basis, Including Loaned Securities	686,414		593,647
Debt Securities, Available-for-sale, Allowance for Credit Loss	0
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax	37,339		14,993
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	(4,791)		(625)
Available-for-sale Securities, Debt Securities, Including Loaned Securities	718,962		608,015
Fixed-maturities available for sale \| Other ABS
Debt Securities, Available-for-sale [Line Items]
Available-for-sale Debt Securities, Amortized Cost Basis, Including Loaned Securities	811,687		760,785
Debt Securities, Available-for-sale, Allowance for Credit Loss	0
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax	4,051		2,018
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	(6,350)		(3,674)
Available-for-sale Securities, Debt Securities, Including Loaned Securities	809,388		759,129
Fixed-maturities available for sale \| Foreign government and agency securities
Debt Securities, Available-for-sale [Line Items]
Available-for-sale Debt Securities, Amortized Cost Basis, Including Loaned Securities	5,098		5,091
Debt Securities, Available-for-sale, Allowance for Credit Loss	0
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax	198		133
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	0		0
Available-for-sale Securities, Debt Securities, Including Loaned Securities	5,296		5,224
Fixed-maturities available for sale \| Total fixed-maturities available for sale
Debt Securities, Available-for-sale [Line Items]
Available-for-sale Debt Securities, Amortized Cost Basis, Including Loaned Securities	5,351,426		4,573,387
Debt Securities, Available-for-sale, Amortized Cost	5,338,536		4,549,534
Debt Securities, Available-for-sale, Allowance for Credit Loss	(2,290)
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax	313,807		147,865
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	(20,496)		(8,328)
Available-for-sale Securities, Debt Securities, Including Loaned Securities	5,642,447		4,712,924
Fixed-maturity investments available for sale, Total, Fair Value	5,628,992		4,688,911
Fixed-maturities available for sale \| Securities Financing Transaction, Cost \| Corporate bonds and notes
Debt Securities, Available-for-sale [Line Items]
Less: loaned securities	12,890
Fixed-maturities available for sale \| Securities Financing Transaction, Fair Value \| Corporate bonds and notes
Debt Securities, Available-for-sale [Line Items]
Less: loaned securities	$ 13,455		$ 24,013

Available-for-sale Debt Securities, Amortized Cost Basis, Including Loaned Securities

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Available-for-sale Securities, Debt Securities, Including Loaned Securities

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Amortized cost of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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Amount before tax of unrealized gain in accumulated other comprehensive income (AOCI) on investments in debt and equity securities classified as available-for-sale.

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Amount of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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Amount of allowance for credit loss on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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