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Fair Value Measurements (Carrying Value and Estimated Fair Value of Financial Instruments) (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Fixed Income Securities, Available-for-sale, at fair value	$ 6,455,928	$ 6,095,620
Equity securities	310,367	72,937
Short-term investments	409,852	282,490
Long-term debt	550,743	550,597
Unamortized Debt Issuance Costs	(3,419)
Finance Lease, Liability	508	737
Seven Point Twenty Five Percent Senior Notes [Member]
Long-term Debt, Fair Value	66,148	66,365
Unamortized Debt Issuance Costs	(166)
Six Point Seventy Percent Senior Notes [Member]
Long-term Debt, Fair Value	127,886	123,104
Unamortized Debt Issuance Costs	(319)
Five Point Three Hundred Seventy Five [Member]
Long-term Debt, Fair Value	383,669	357,025
Unamortized Debt Issuance Costs	(2,934)
One Point Sixty One Percent Borrowing From Federal Home Loan Bank Due July 21, 2021 [Member]
Long-term Debt, Fair Value	25,182	24,901
Unamortized Debt Issuance Costs	0
One Point Fifty Six Percent Borrowing From Federal Home Loan Bank Due August 16, 2021 [Member]
Long-term Debt, Fair Value	25,198	24,875
Unamortized Debt Issuance Costs	0
Three Point Zero Three Percent Borrowing From Federal Home Loan Bank Due December 16, 2026 [Member]
Long-term Debt, Fair Value	67,513	63,002
Unamortized Debt Issuance Costs	0
Finance lease obligation [Member]
Finance Lease, Liability	508	737
Carrying Amount [Member]
Long-term debt	550,743	550,597
Carrying Amount [Member] | Seven Point Twenty Five Percent Senior Notes [Member]
Long-term debt	49,914	49,910
Carrying Amount [Member] | Six Point Seventy Percent Senior Notes [Member]
Long-term debt	99,499	99,480
Carrying Amount [Member] | Five Point Three Hundred Seventy Five [Member]
Long-term debt	294,241	294,157
Carrying Amount [Member] | One Point Sixty One Percent Borrowing From Federal Home Loan Bank Due July 21, 2021 [Member]
Long-term debt	25,000	25,000
Carrying Amount [Member] | One Point Fifty Six Percent Borrowing From Federal Home Loan Bank Due August 16, 2021 [Member]
Long-term debt	25,000	25,000
Carrying Amount [Member] | Three Point Zero Three Percent Borrowing From Federal Home Loan Bank Due December 16, 2026 [Member]
Long-term debt	60,000	60,000
Carrying Amount [Member] | Long Term Debt Excluding Issuance Costs [Member]
Long-term debt	553,654	553,547
Carrying Amount [Member] | Unamortized Debt Issuance Costs [Member]
Unamortized Debt Issuance Costs	(3,419)	(3,687)
Carrying Amount [Member] | Finance lease obligation [Member]
Finance Lease, Liability	508	737
Fair Value [Member]
Long-term Debt, Fair Value	695,596	659,272
Fair Value [Member] | Seven Point Twenty Five Percent Senior Notes [Member]
Long-term Debt, Fair Value	66,148	66,365
Fair Value [Member] | Six Point Seventy Percent Senior Notes [Member]
Long-term Debt, Fair Value	127,886	123,104
Fair Value [Member] | Five Point Three Hundred Seventy Five [Member]
Long-term Debt, Fair Value	383,669	357,025
Fair Value [Member] | One Point Sixty One Percent Borrowing From Federal Home Loan Bank Due July 21, 2021 [Member]
Long-term Debt, Fair Value	25,182	24,901
Fair Value [Member] | One Point Fifty Six Percent Borrowing From Federal Home Loan Bank Due August 16, 2021 [Member]
Long-term Debt, Fair Value	25,198	24,875
Fair Value [Member] | Three Point Zero Three Percent Borrowing From Federal Home Loan Bank Due December 16, 2026 [Member]
Long-term Debt, Fair Value	$ 67,513	$ 63,002

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