Schedule I - Summary of Investments - Other Than Investments in Related Parties (Detail) $ in Thousands	Dec. 31, 2018 USD ($)
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Cost or Amortized Cost	$ 21,640,560
Estimated Fair Value	21,656,362
Amount at Which Shown in the Balance Sheet	22,302,564
Options
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Cost or Amortized Cost	108,803
Estimated Fair Value	148,006
Amount at Which Shown in the Balance Sheet	1,142
Consumer Goods | Common Stock
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Cost or Amortized Cost	148,635
Estimated Fair Value	322,934
Amount at Which Shown in the Balance Sheet	322,934
Energy and Utilities | Common Stock
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Cost or Amortized Cost	91,725
Estimated Fair Value	121,756
Amount at Which Shown in the Balance Sheet	121,756
Finance | Common Stock
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Cost or Amortized Cost	127,396
Estimated Fair Value	259,918
Amount at Which Shown in the Balance Sheet	259,918
Healthcare | Common Stock
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Cost or Amortized Cost	68,731
Estimated Fair Value	206,399
Amount at Which Shown in the Balance Sheet	206,399
Industrials | Common Stock
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Cost or Amortized Cost	43,871
Estimated Fair Value	136,601
Amount at Which Shown in the Balance Sheet	136,601
Information Technology | Common Stock
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Cost or Amortized Cost	118,495
Estimated Fair Value	346,384
Amount at Which Shown in the Balance Sheet	346,384
Other Equity Securities [Member] | Common Stock
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Cost or Amortized Cost	95,669
Estimated Fair Value	115,193
Amount at Which Shown in the Balance Sheet	115,193
Preferred Stock
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Cost or Amortized Cost	19,982
Estimated Fair Value	21,042
Amount at Which Shown in the Balance Sheet	21,042
Mortgage Loans
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Cost or Amortized Cost	5,124,707
Estimated Fair Value	5,049,468
Amount at Which Shown in the Balance Sheet	5,124,707
Real Estate
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Cost or Amortized Cost	544,823
Estimated Fair Value	0
Amount at Which Shown in the Balance Sheet	544,823
Real Estate Acquired in Satisfaction of Debt
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Cost or Amortized Cost	42,694
Estimated Fair Value	0
Amount at Which Shown in the Balance Sheet	42,694
Policy Loans
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Cost or Amortized Cost	376,254
Estimated Fair Value	376,254
Amount at Which Shown in the Balance Sheet	376,254
Other Long-term Investments
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Cost or Amortized Cost	48,945
Estimated Fair Value	0
Amount at Which Shown in the Balance Sheet	48,945
Short-term Investments
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Cost or Amortized Cost	206,760
Estimated Fair Value	206,760
Amount at Which Shown in the Balance Sheet	206,760
Held-to-maturity Securities | Foreign Governments
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Cost or Amortized Cost	3,961
Estimated Fair Value	4,430
Amount at Which Shown in the Balance Sheet	3,961
Held-to-maturity Securities | Corporate Debt Securities
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Cost or Amortized Cost	7,640,891
Estimated Fair Value	7,548,829
Amount at Which Shown in the Balance Sheet	7,640,891
Held-to-maturity Securities | U.S. States and Political Subdivisions
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Cost or Amortized Cost	245,360
Estimated Fair Value	250,899
Amount at Which Shown in the Balance Sheet	245,360
Held-to-maturity Securities | Residential Mortgage-Backed Securities
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Cost or Amortized Cost	315,306
Estimated Fair Value	319,910
Amount at Which Shown in the Balance Sheet	315,306
Held-to-maturity Securities | Collateralized Debt Securities
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Cost or Amortized Cost	5,214
Estimated Fair Value	5,285
Amount at Which Shown in the Balance Sheet	5,214
Held-to-maturity Securities | Other Debt Securities
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Cost or Amortized Cost	717
Estimated Fair Value	731
Amount at Which Shown in the Balance Sheet	717
Available-for-sale Securities | Foreign Governments
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Cost or Amortized Cost	5,000
Estimated Fair Value	6,210
Amount at Which Shown in the Balance Sheet	6,210
Available-for-sale Securities | Corporate Debt Securities
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Cost or Amortized Cost	5,345,579
Estimated Fair Value	5,283,818
Amount at Which Shown in the Balance Sheet	5,283,818
Available-for-sale Securities | U.S. States and Political Subdivisions
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Cost or Amortized Cost	848,228
Estimated Fair Value	862,030
Amount at Which Shown in the Balance Sheet	862,030
Available-for-sale Securities | Residential Mortgage-Backed Securities
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Cost or Amortized Cost	31,735
Estimated Fair Value	31,662
Amount at Which Shown in the Balance Sheet	31,662
Available-for-sale Securities | Collateralized Debt Securities
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Cost or Amortized Cost	2,775
Estimated Fair Value	3,444
Amount at Which Shown in the Balance Sheet	3,444
Available-for-sale Securities | U.S. Treasury and Government
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Cost or Amortized Cost	28,304
Estimated Fair Value	28,399
Amount at Which Shown in the Balance Sheet	$ 28,399