Note 6 - Investments - Contractual Maturities (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Available-for-sale Securities, Amortized Cost
Fixed-maturities, Available-for-sale, Amortized Cost	$ 5,393,623	$ 4,549,534
Available-for-sale Securities, Fair Value
Fixed-maturities available for sale—at fair value	5,723,340	4,688,911
Securities Loaned, Asset	57,499	66,442
Fixed-maturities available for sale \| Non Asset Backed Security Investments, Contractual Maturities
Available-for-sale Securities, Amortized Cost
Due in one year or less	125,361
Due after one year through five years	1,002,786
Due after five years through ten years	1,225,712
Due after ten years	808,882
Available-for-sale Securities, Fair Value
Due in one year or less	126,414
Due after one year through five years	1,062,579
Due after five years through ten years	1,338,147
Due after ten years	896,083
Fixed-maturities available for sale \| RMBS
Available-for-sale Securities, Fair Value
Available-for-sale Debt Securities, Amortized Cost Basis, Including Loaned Securities	799,814	766,429
Available-for-sale Securities, Debt Securities, Including Loaned Securities	833,939	779,354
Fixed-maturities available for sale \| CMBS
Available-for-sale Securities, Fair Value
Available-for-sale Debt Securities, Amortized Cost Basis, Including Loaned Securities	645,071	574,037
Available-for-sale Securities, Debt Securities, Including Loaned Securities	681,265	588,420
Fixed-maturities available for sale \| Other ABS
Available-for-sale Securities, Fair Value
Available-for-sale Debt Securities, Amortized Cost Basis, Including Loaned Securities	249,988	314,946
Available-for-sale Securities, Debt Securities, Including Loaned Securities	252,457	316,163
Fixed-maturities available for sale \| Total fixed-maturities available for sale
Available-for-sale Securities, Amortized Cost
Fixed-maturities, Available-for-sale, Amortized Cost	5,393,623	4,549,534
Available-for-sale Securities, Fair Value
Fixed-maturities available for sale—at fair value	5,723,340	4,688,911
Available-for-sale Debt Securities, Amortized Cost Basis, Including Loaned Securities	5,426,787	4,573,387
Available-for-sale Securities, Debt Securities, Including Loaned Securities	5,759,442	4,712,924
Fixed-maturities available for sale \| Asset-backed and other Mortgage Backed securities [Member]
Available-for-sale Securities, Amortized Cost
Debt Securities, Available-for-sale, Maturity, without Single Maturity Date, Amortized Cost	2,264,046
Available-for-sale Securities, Fair Value
Debt Securities, Available-for-sale, Maturity, without Single Maturity Date, Fair Value	2,336,219
Securities Financing Transaction, Amortized Cost \| Fixed-maturities available for sale \| Total fixed-maturities available for sale
Available-for-sale Securities, Fair Value
Securities Loaned, Asset	33,164	23,853
Securities Financing Transaction, Fair Value \| Fixed-maturities available for sale
Available-for-sale Securities, Fair Value
Securities Loaned, Asset	36,100
Securities Financing Transaction, Fair Value \| Fixed-maturities available for sale \| Total fixed-maturities available for sale
Available-for-sale Securities, Fair Value
Securities Loaned, Asset	$ 36,102	$ 24,013

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