Schedule I - Summary of Investments Other Than Investments in Related Parties (Details)	Dec. 31, 2021 USD ($)
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Amortized Cost or Cost	$ 10,484,377,000
Balance Sheet Amount	10,972,071,000
Fixed maturities:
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Amortized Cost or Cost	9,670,358,000
Fair Value	10,146,384,000
Balance Sheet Amount	10,146,384,000
Equity securities
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Amortized Cost or Cost	16,549,000
Fair Value	28,217,000
Balance Sheet Amount	28,217,000
Public utilities
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Amortized Cost or Cost	578,000
Fair Value	1,335,000
Balance Sheet Amount	1,335,000
Banks, trust, and insurance companies
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Amortized Cost or Cost	1,701,000
Fair Value	4,287,000
Balance Sheet Amount	4,287,000
Industrial, miscellaneous, and all others
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Amortized Cost or Cost	9,945,000
Fair Value	18,173,000
Balance Sheet Amount	18,173,000
Preferred stocks
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Amortized Cost or Cost	4,325,000
Fair Value	4,422,000
Balance Sheet Amount	4,422,000
Derivatives, index options
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Amortized Cost or Cost	101,622,000
Balance Sheet Amount	101,622,000
Mortgage loans on real estate
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Amortized Cost or Cost	486,892,000
Balance Sheet Amount	486,892,000
Policy loans
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Amortized Cost or Cost	71,286,000
Balance Sheet Amount	71,286,000
Other long-term investments
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Amortized Cost or Cost	137,670,000
Balance Sheet Amount	137,670,000
Real estate acquired through foreclosure	0
Debt securities available-for-sale:
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Amortized Cost or Cost	8,604,250,000
Fair Value	9,068,946,000
Balance Sheet Amount	9,068,946,000
Debt securities available-for-sale: | United States government and government agencies and authorities
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Amortized Cost or Cost	45,941,000
Fair Value	47,033,000
Balance Sheet Amount	47,033,000
Debt securities available-for-sale: | States, municipalities, and political subdivisions
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Amortized Cost or Cost	479,148,000
Fair Value	505,960,000
Balance Sheet Amount	505,960,000
Debt securities available-for-sale: | Foreign governments
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Amortized Cost or Cost	62,979,000
Fair Value	62,391,000
Balance Sheet Amount	62,391,000
Debt securities available-for-sale: | Public utilities
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Amortized Cost or Cost	745,359,000
Fair Value	784,969,000
Balance Sheet Amount	784,969,000
Debt securities available-for-sale: | Corporate
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Amortized Cost or Cost	6,322,471,000
Fair Value	6,700,953,000
Balance Sheet Amount	6,700,953,000
Debt securities available-for-sale: | Commercial mortgage-backed
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Amortized Cost or Cost	27,016,000
Fair Value	27,757,000
Balance Sheet Amount	27,757,000
Debt securities available-for-sale: | Residential mortgage-backed
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Amortized Cost or Cost	530,702,000
Fair Value	549,623,000
Balance Sheet Amount	549,623,000
Debt securities available-for-sale: | Asset-backed
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Amortized Cost or Cost	390,634,000
Fair Value	390,260,000
Balance Sheet Amount	390,260,000
Debt securities trading:
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Amortized Cost or Cost	1,066,108,000
Fair Value	1,077,438,000
Balance Sheet Amount	1,077,438,000
Debt securities trading: | United States government and government agencies and authorities
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Amortized Cost or Cost	0
Fair Value	0
Balance Sheet Amount	0
Debt securities trading: | States, municipalities, and political subdivisions
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Amortized Cost or Cost	16,644,000
Fair Value	17,487,000
Balance Sheet Amount	17,487,000
Debt securities trading: | Foreign governments
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Amortized Cost or Cost	0
Fair Value	0
Balance Sheet Amount	0
Debt securities trading: | Public utilities
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Amortized Cost or Cost	36,282,000
Fair Value	36,973,000
Balance Sheet Amount	36,973,000
Debt securities trading: | Corporate
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Amortized Cost or Cost	413,250,000
Fair Value	423,778,000
Balance Sheet Amount	423,778,000
Debt securities trading: | Commercial mortgage-backed
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Amortized Cost or Cost	240,484,000
Fair Value	240,573,000
Balance Sheet Amount	240,573,000
Debt securities trading: | Residential mortgage-backed
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Amortized Cost or Cost	45,195,000
Fair Value	44,772,000
Balance Sheet Amount	44,772,000
Debt securities trading: | Asset-backed
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Amortized Cost or Cost	314,253,000
Fair Value	313,855,000
Balance Sheet Amount	$ 313,855,000