Schedule I Summary of Investments-Other Than Investments in Related Parties (Details) $ in Thousands	Dec. 31, 2019 USD ($)
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Cost	$ 6,299,955
Amount Shown in Balance Sheet	6,639,233
Fixed maturity securities
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Cost	5,456,980
Fair Value	5,791,676
Amount Shown in Balance Sheet	5,791,676
Equity securities
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Cost	101,864
Fair Value	101,864
Amount Shown in Balance Sheet	101,864
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	906,886
Fair Value	956,104
Amount Shown in Balance Sheet	956,104
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	1,545,787
Fair Value	1,686,203
Amount Shown in Balance Sheet	1,686,203
Foreign government bonds | Fixed maturity securities
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Cost	42,801
Fair Value	45,370
Amount Shown in Balance Sheet	45,370
Public utilities | Fixed maturity securities
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Cost	54,519
Fair Value	61,748
Amount Shown in Balance Sheet	61,748
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	1,388,035
Fair Value	1,495,426
Amount Shown in Balance Sheet	1,495,426
Asset-backed securities | Fixed maturity securities
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Cost	1,068,661
Fair Value	1,079,586
Amount Shown in Balance Sheet	1,079,586
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	76,237
Fair Value	75,933
Amount Shown in Balance Sheet	75,933
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	352,164
Fair Value	367,055
Amount Shown in Balance Sheet	367,055
Redeemable preferred stocks | Fixed maturity securities
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Cost	21,890
Fair Value	24,251
Amount Shown in Balance Sheet	24,251
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	13,360
Fair Value	13,360
Amount Shown in Balance Sheet	13,360
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	5,706
Fair Value	5,706
Amount Shown in Balance Sheet	5,706
Public utilities | Equity securities
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Cost	1,052
Fair Value	1,052
Amount Shown in Balance Sheet	1,052
Non-redeemable preferred stocks | Equity securities
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Cost	60,325
Fair Value	60,325
Amount Shown in Balance Sheet	60,325
Closed-end fund | Equity securities
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Cost	21,421
Fair Value	21,421
Amount Shown in Balance Sheet	21,421
Limited partnership interests
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Cost	383,717
Amount Shown in Balance Sheet	383,717
Short-term Investments
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Cost	172,667
Amount Shown in Balance Sheet	172,667
Policy loans
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Cost	153,541
Amount Shown in Balance Sheet	153,541
Derivatives
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Cost	8,657
Fair Value	13,239
Amount Shown in Balance Sheet	13,239
Mortgage loans
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Cost	9,771
Amount Shown in Balance Sheet	9,771
Other
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Cost	12,758
Amount Shown in Balance Sheet	$ 12,758