Schedule I Summary of Investments-Other Than Investments in Related Parties (Details) $ in Thousands	Dec. 31, 2017 USD ($)
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	$ 7,901,923
Amount Shown in Balance Sheet	8,352,348
Fixed maturity securities
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	7,302,950
Fair Value	7,724,075
Amount Shown in Balance Sheet	7,724,075
Equity securities
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	116,320
Fair Value	135,466
Amount Shown in Balance Sheet	135,466
U.S. Government and federally sponsored agency obligations | Fixed maturity securities
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	1,131,962
Fair Value	1,177,760
Amount Shown in Balance Sheet	1,177,760
States, municipalities and political subdivisions | Fixed maturity securities
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	1,711,581
Fair Value	1,893,252
Amount Shown in Balance Sheet	1,893,252
Foreign government bonds | Fixed maturity securities
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	96,780
Fair Value	102,738
Amount Shown in Balance Sheet	102,738
Public utilities | Fixed maturity securities
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	107,339
Fair Value	125,201
Amount Shown in Balance Sheet	125,201
All other corporate bonds | Fixed maturity securities
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	2,284,448
Fair Value	2,433,349
Amount Shown in Balance Sheet	2,433,349
Asset-backed securities | Fixed maturity securities
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	1,285,804
Fair Value	1,301,634
Amount Shown in Balance Sheet	1,301,634
Residential mortgage-backed securities (non-agency) | Fixed maturity securities
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	86,741
Fair Value	87,773
Amount Shown in Balance Sheet	87,773
Commercial mortgage-backed securities | Fixed maturity securities
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	580,655
Fair Value	581,962
Amount Shown in Balance Sheet	581,962
Redeemable preferred stocks | Fixed maturity securities
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	17,640
Fair Value	20,406
Amount Shown in Balance Sheet	20,406
Industrial, miscellaneous and all other | Equity securities
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	26,040
Fair Value	34,623
Amount Shown in Balance Sheet	34,623
Banking & finance and insurance companies | Equity securities
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	10,601
Fair Value	16,979
Amount Shown in Balance Sheet	16,979
Public utilities | Equity securities
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	1,104
Fair Value	1,844
Amount Shown in Balance Sheet	1,844
Non-redeemable preferred stocks | Equity securities
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	58,571
Fair Value	61,458
Amount Shown in Balance Sheet	61,458
Closed-end fund | Equity securities
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	20,004
Fair Value	20,562
Amount Shown in Balance Sheet	20,562
Short-term Investments
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	62,593
Amount Shown in Balance Sheet	62,593
Policy loans
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	153,635
Amount Shown in Balance Sheet	153,635
Derivative instruments
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	5,396
Amount Shown in Balance Sheet	15,550
Mortgage loans
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	1,263
Amount Shown in Balance Sheet	1,263
Other
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	259,766
Amount Shown in Balance Sheet	$ 259,766