Schedule I - Summary of Investments - Other Than Investments in Related Parties (Detail) $ in Thousands	Dec. 31, 2019 USD ($)
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Cost or amortized cost	$ 22,361,282
Estimated fair value	23,764,723
Amount at which shown in the balance sheet	23,587,089
Options
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Cost or amortized cost	103,518
Estimated fair value	256,005
Amount at which shown in the balance sheet	2,008
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	121,124
Estimated fair value	320,444
Amount at which shown in the balance sheet	320,444
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	94,916
Estimated fair value	132,700
Amount at which shown in the balance sheet	132,700
Finance | Common Stock
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Cost or amortized cost	116,653
Estimated fair value	291,696
Amount at which shown in the balance sheet	291,696
Healthcare | Common Stock
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Cost or amortized cost	70,693
Estimated fair value	221,319
Amount at which shown in the balance sheet	221,319
Industrials | Common Stock
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Cost or amortized cost	35,245
Estimated fair value	127,809
Amount at which shown in the balance sheet	127,809
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	99,502
Estimated fair value	426,682
Amount at which shown in the balance sheet	426,682
Other | Common Stock
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Cost or amortized cost	107,667
Estimated fair value	161,036
Amount at which shown in the balance sheet	161,036
Preferred Stock
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Cost or amortized cost	17,258
Estimated fair value	19,274
Amount at which shown in the balance sheet	19,274
Mortgage Loans
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Cost or amortized cost	5,097,017
Estimated fair value	5,309,005
Amount at which shown in the balance sheet	5,097,017
Real Estate
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Cost or amortized cost	513,419
Estimated fair value	0
Amount at which shown in the balance sheet	513,419
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	37,800
Estimated fair value	0
Amount at which shown in the balance sheet	37,800
Policy Loans
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Cost or amortized cost	379,657
Estimated fair value	379,657
Amount at which shown in the balance sheet	379,657
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	74,561
Estimated fair value	0
Amount at which shown in the balance sheet	74,561
Short-term Investments
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Cost or amortized cost	425,321
Estimated fair value	425,321
Amount at which shown in the balance sheet	425,321
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	3,907
Estimated fair value	4,349
Amount at which shown in the balance sheet	3,907
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	8,068,966
Estimated fair value	8,393,788
Amount at which shown in the balance sheet	8,068,966
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	195,241
Estimated fair value	201,295
Amount at which shown in the balance sheet	195,241
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	237,516
Estimated fair value	242,828
Amount at which shown in the balance sheet	237,516
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	125,631
Estimated fair value	126,430
Amount at which shown in the balance sheet	125,631
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	6,287
Amount at which shown in the balance sheet	6,287
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	5,320,990
Estimated fair value	5,558,684
Amount at which shown in the balance sheet	5,558,684
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	1,047,326
Estimated fair value	1,096,101
Amount at which shown in the balance sheet	1,096,101
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	23,405
Estimated fair value	23,943
Amount at which shown in the balance sheet	23,943
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	9,444
Estimated fair value	10,129
Amount at which shown in the balance sheet	10,129
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	29,505
Estimated fair value	29,941
Amount at which shown in the balance sheet	$ 29,941