Fair Value of Financial Instruments - Carrying Values and Estimated Fair Values of Financial Instruments (Details) - USD ($) $ in Thousands		Jun. 30, 2021	Dec. 31, 2020
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Available-for-sale debt securities, at fair value		$ 2,514,585	$ 2,464,611
Fixed-maturity security held to maturity, fair value		1,572,889	1,606,208
Short-term investments available-for-sale		40,325
Equity securities		39,377	38,023
Separate account assets		2,745,827	2,659,520
Surplus note		1,368,194	1,345,772
Separate account liabilities		2,745,827	2,659,520
Fixed-maturity security (held-to-maturity)		1,368,740	1,346,350
Reported Value Measurement [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Available-for-sale debt securities, at fair value		2,514,585	2,464,611
Short-term investments available-for-sale		40,327
Equity securities		39,377	38,023
Trading securities		33,498	16,300
Separate account assets		2,745,827	2,659,520
Separate account liabilities		2,745,827	2,659,520
Reported Value Measurement [Member] | Level 3 [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Policy loans	[1]	29,449	30,199
Surplus note	[1],[2]	1,368,194	1,345,772
Fixed-maturity security (held-to-maturity)	[1]	1,368,740	1,346,350
Reported Value Measurement [Member] | Level 3 [Member] | Deposits [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Deposit asset underlying 10% coinsurance agreement	[1]	232,293	236,865
Reported Value Measurement [Member] | Senior Notes | Level 2 [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Notes payable – Long term	[2],[3]	374,606	374,415
Estimate of Fair Value Measurement [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Available-for-sale debt securities, at fair value		2,514,585	2,464,611
Short-term investments available-for-sale		40,325
Equity securities		39,377	38,023
Trading securities		33,498	16,300
Separate account assets		2,745,827	2,659,520
Separate account liabilities		2,745,827	2,659,520
Estimate of Fair Value Measurement [Member] | Level 3 [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Fixed-maturity security held to maturity, fair value	[1]	1,572,889	1,606,208
Policy loans	[1]	29,449	30,199
Surplus note	[1],[2]	1,564,889	1,596,599
Estimate of Fair Value Measurement [Member] | Level 3 [Member] | Deposits [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Deposit asset underlying 10% coinsurance agreement	[1]	232,293	236,865
Estimate of Fair Value Measurement [Member] | Senior Notes | Level 2 [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Notes payable – Long term	[2],[3]	$ 390,953	$ 399,377

[1] Classified as a Level 3 fair value measurement.

[2] Carrying value amounts shown are net of issuance costs

[3] Classified as a Level 2 fair value measurement.