Schedule I Summary of Investments-Other Than Investments in Related Parties (Details) $ in Thousands	Dec. 31, 2020 USD ($)
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Cost	$ 6,698,328
Amount Shown in Balance Sheet	7,262,234
Fixed maturity securities
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Cost	5,788,551
Fair Value	6,345,290
Amount Shown in Balance Sheet	6,345,290
Equity securities
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Cost	121,653
Fair Value	121,653
Amount Shown in Balance Sheet	121,653
U.S. Government and federally sponsored agency obligations | Fixed maturity securities
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Cost	814,204
Fair Value	901,517
Amount Shown in Balance Sheet	901,517
States, municipalities and political subdivisions | Fixed maturity securities
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Cost	1,612,290
Fair Value	1,827,497
Amount Shown in Balance Sheet	1,827,497
Foreign government bonds | Fixed maturity securities
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Cost	40,145
Fair Value	45,053
Amount Shown in Balance Sheet	45,053
Public utilities | Fixed maturity securities
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Cost	64,898
Fair Value	76,382
Amount Shown in Balance Sheet	76,382
All other corporate bonds | Fixed maturity securities
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Cost	1,811,755
Fair Value	2,015,192
Amount Shown in Balance Sheet	2,015,192
Asset-backed securities | Fixed maturity securities
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Cost	1,053,252
Fair Value	1,051,258
Amount Shown in Balance Sheet	1,051,258
Residential mortgage-backed securities (non-agency) | Fixed maturity securities
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Cost	53,727
Fair Value	53,627
Amount Shown in Balance Sheet	53,627
Commercial mortgage-backed securities | Fixed maturity securities
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Cost	309,726
Fair Value	343,439
Amount Shown in Balance Sheet	343,439
Redeemable preferred stocks | Fixed maturity securities
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Cost	28,554
Fair Value	31,325
Amount Shown in Balance Sheet	31,325
Industrial, miscellaneous and all other | Equity securities
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Cost	5,645
Fair Value	5,645
Amount Shown in Balance Sheet	5,645
Banking & finance and insurance companies | Equity securities
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Cost	1,852
Fair Value	1,852
Amount Shown in Balance Sheet	1,852
Public utilities | Equity securities
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Cost	388
Fair Value	388
Amount Shown in Balance Sheet	388
Non-redeemable preferred stocks | Equity securities
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Cost	90,709
Fair Value	90,709
Amount Shown in Balance Sheet	90,709
Closed-end fund | Equity securities
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Cost	23,059
Fair Value	23,059
Amount Shown in Balance Sheet	23,059
Limited partnership interests
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Cost	448,996
Amount Shown in Balance Sheet	448,996
Short-term Investments
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Cost	141,770
Amount Shown in Balance Sheet	141,770
Policy loans
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Cost	150,118
Amount Shown in Balance Sheet	150,118
Derivatives
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Cost	9,638
Fair Value	16,805
Amount Shown in Balance Sheet	16,805
Mortgage loans
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Cost	18,150
Amount Shown in Balance Sheet	18,150
Other
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Cost	19,452
Amount Shown in Balance Sheet	$ 19,452