Fair Value of Financial Instruments - Quantitative Disclosures Regarding Fair Value Hierarchy Measurements (Detail) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Financial assets
Fixed maturity securities, bonds available-for-sale	$ 6,826,245	$ 6,215,563
Equity securities	1,678,657	1,530,228
Short-term investments	398,948	206,760
Separate account assets	1,025,370	918,369
Fixed maturity securities, bonds held-to-maturity	8,715,569	8,211,449
Fair value of bonds held-to-maturity	9,063,688	8,130,084
Mortgage loans on real estate, net allowance	4,955,540	5,146,040
Policy loans	380,018	376,254
Total financial assets	28,518,519	26,912,353
Financial liabilities
Separate account liabilities	1,025,370	918,369
Notes payable	159,043	137,963
Total financial liabilities	22,686,811	21,640,838
U.S. treasury and government
Financial assets
Fixed maturity securities, bonds available-for-sale	29,961	28,399
U.S. states and political subdivisions
Financial assets
Fixed maturity securities, bonds available-for-sale	1,105,745	862,030
Fixed maturity securities, bonds held-to-maturity	203,858	245,360
Fair value of bonds held-to-maturity	210,827	250,899
Foreign governments
Financial assets
Fixed maturity securities, bonds available-for-sale	6,361	6,210
Fixed maturity securities, bonds held-to-maturity	3,921	3,961
Fair value of bonds held-to-maturity	4,390	4,430
Corporate debt securities
Financial assets
Fixed maturity securities, bonds available-for-sale	5,649,350	5,283,818
Fixed maturity securities, bonds held-to-maturity	8,152,982	7,640,891
Fair value of bonds held-to-maturity	8,483,944	7,548,829
Residential mortgage-backed securities
Financial assets
Fixed maturity securities, bonds available-for-sale	24,464	31,662
Fixed maturity securities, bonds held-to-maturity	208,752	315,306
Fair value of bonds held-to-maturity	216,008	319,910
Collateralized debt securities
Financial assets
Fixed maturity securities, bonds available-for-sale	10,364	3,444
Fixed maturity securities, bonds held-to-maturity	146,056	5,214
Fair value of bonds held-to-maturity	148,519	5,285
Other debt securities
Financial assets
Fixed maturity securities, bonds held-to-maturity		717
Fair value of bonds held-to-maturity		731
Recurring
Financial assets
Fixed maturity securities, bonds available-for-sale	6,826,245	6,215,563
Equity securities	1,678,657	1,530,228
Equity-indexed options	224,184	148,006
Short-term investments	398,948	206,760
Separate account assets	1,008,309	905,824
Separately managed accounts	40,093	16,532
Total financial assets	10,176,436	9,022,913
Financial liabilities
Embedded derivative liability for equity-indexed contracts	691,766	596,075
Separate account liabilities	1,008,309	905,824
Total financial liabilities	1,700,075	1,501,899
Recurring | Common stock
Financial assets
Equity securities	1,657,070	1,509,186
Recurring | Preferred stock
Financial assets
Equity securities	21,587	21,042
Recurring | U.S. treasury and government
Financial assets
Fixed maturity securities, bonds available-for-sale	29,961	28,399
Recurring | U.S. states and political subdivisions
Financial assets
Fixed maturity securities, bonds available-for-sale	1,105,745	862,030
Recurring | Foreign governments
Financial assets
Fixed maturity securities, bonds available-for-sale	6,361	6,210
Recurring | Corporate debt securities
Financial assets
Fixed maturity securities, bonds available-for-sale	5,649,350	5,283,818
Recurring | Residential mortgage-backed securities
Financial assets
Fixed maturity securities, bonds available-for-sale	24,464	31,662
Recurring | Collateralized debt securities
Financial assets
Fixed maturity securities, bonds available-for-sale	10,364	3,444
Recurring | Level 1
Financial assets
Fixed maturity securities, bonds available-for-sale	0	0
Equity securities	1,678,194	1,530,115
Equity-indexed options	0	0
Short-term investments	0	0
Separate account assets	253,925	227,448
Separately managed accounts	0	0
Total financial assets	1,932,119	1,757,563
Financial liabilities
Embedded derivative liability for equity-indexed contracts	0	0
Separate account liabilities	253,925	227,448
Total financial liabilities	253,925	227,448
Recurring | Level 1 | Common stock
Financial assets
Equity securities	1,657,070	1,509,073
Recurring | Level 1 | Preferred stock
Financial assets
Equity securities	21,124	21,042
Recurring | Level 1 | U.S. treasury and government
Financial assets
Fixed maturity securities, bonds available-for-sale	0	0
Recurring | Level 1 | U.S. states and political subdivisions
Financial assets
Fixed maturity securities, bonds available-for-sale	0	0
Recurring | Level 1 | Foreign governments
Financial assets
Fixed maturity securities, bonds available-for-sale	0	0
Recurring | Level 1 | Corporate debt securities
Financial assets
Fixed maturity securities, bonds available-for-sale	0	0
Recurring | Level 1 | Residential mortgage-backed securities
Financial assets
Fixed maturity securities, bonds available-for-sale	0	0
Recurring | Level 1 | Collateralized debt securities
Financial assets
Fixed maturity securities, bonds available-for-sale	0	0
Recurring | Level 2
Financial assets
Fixed maturity securities, bonds available-for-sale	6,822,012	6,211,330
Equity securities	0	0
Equity-indexed options	0	0
Short-term investments	398,948	206,760
Separate account assets	754,384	678,376
Separately managed accounts	0	0
Total financial assets	7,975,344	7,096,466
Financial liabilities
Embedded derivative liability for equity-indexed contracts	0	0
Separate account liabilities	754,384	678,376
Total financial liabilities	754,384	678,376
Recurring | Level 2 | Common stock
Financial assets
Equity securities	0	0
Recurring | Level 2 | Preferred stock
Financial assets
Equity securities	0	0
Recurring | Level 2 | U.S. treasury and government
Financial assets
Fixed maturity securities, bonds available-for-sale	29,961	28,399
Recurring | Level 2 | U.S. states and political subdivisions
Financial assets
Fixed maturity securities, bonds available-for-sale	1,105,745	862,030
Recurring | Level 2 | Foreign governments
Financial assets
Fixed maturity securities, bonds available-for-sale	6,361	6,210
Recurring | Level 2 | Corporate debt securities
Financial assets
Fixed maturity securities, bonds available-for-sale	5,645,117	5,279,585
Recurring | Level 2 | Residential mortgage-backed securities
Financial assets
Fixed maturity securities, bonds available-for-sale	24,464	31,662
Recurring | Level 2 | Collateralized debt securities
Financial assets
Fixed maturity securities, bonds available-for-sale	10,364	3,444
Recurring | Level 3
Financial assets
Fixed maturity securities, bonds available-for-sale	4,233	4,233
Equity securities	463	113
Equity-indexed options	224,184	148,006
Short-term investments	0	0
Separate account assets	0	0
Separately managed accounts	40,093	16,532
Total financial assets	268,973	168,884
Financial liabilities
Embedded derivative liability for equity-indexed contracts	691,766	596,075
Separate account liabilities	0	0
Total financial liabilities	691,766	596,075
Recurring | Level 3 | Common stock
Financial assets
Equity securities	0	113
Recurring | Level 3 | Preferred stock
Financial assets
Equity securities	463	0
Recurring | Level 3 | U.S. treasury and government
Financial assets
Fixed maturity securities, bonds available-for-sale	0	0
Recurring | Level 3 | U.S. states and political subdivisions
Financial assets
Fixed maturity securities, bonds available-for-sale	0	0
Recurring | Level 3 | Foreign governments
Financial assets
Fixed maturity securities, bonds available-for-sale	0	0
Recurring | Level 3 | Corporate debt securities
Financial assets
Fixed maturity securities, bonds available-for-sale	4,233	4,233
Recurring | Level 3 | Residential mortgage-backed securities
Financial assets
Fixed maturity securities, bonds available-for-sale	0	0
Recurring | Level 3 | Collateralized debt securities
Financial assets
Fixed maturity securities, bonds available-for-sale	0	0
Nonrecurring
Financial assets
Fixed maturity securities, bonds held-to-maturity	8,715,569	8,211,449
Fair value of bonds held-to-maturity	9,063,688	8,130,084
Total financial assets	14,032,192	13,712,410
Total financial assets	14,499,926	13,555,806
Financial liabilities
Total financial liabilities	10,553,559	10,141,953
Total financial liabilities	10,553,559	10,141,953
Nonrecurring | Level 2 | U.S. states and political subdivisions
Financial assets
Fixed maturity securities, bonds held-to-maturity	203,858	245,360
Fair value of bonds held-to-maturity	210,827	250,899
Nonrecurring | Level 2 | Foreign governments
Financial assets
Fixed maturity securities, bonds held-to-maturity	3,921	3,961
Fair value of bonds held-to-maturity	4,390	4,430
Nonrecurring | Level 2 | Corporate debt securities
Financial assets
Fixed maturity securities, bonds held-to-maturity	8,148,982	7,640,891
Fair value of bonds held-to-maturity	8,479,945	7,548,829
Nonrecurring | Level 2 | Residential mortgage-backed securities
Financial assets
Fixed maturity securities, bonds held-to-maturity	208,752	315,306
Fair value of bonds held-to-maturity	216,007	319,910
Nonrecurring | Level 2 | Collateralized debt securities
Financial assets
Fixed maturity securities, bonds held-to-maturity	146,056	5,214
Fair value of bonds held-to-maturity	148,519	5,285
Nonrecurring | Level 2 | Other debt securities
Financial assets
Fixed maturity securities, bonds held-to-maturity		717
Fair value of bonds held-to-maturity		731
Nonrecurring | Level 3
Financial assets
Mortgage loans on real estate, net allowance	4,936,605	5,124,707
Mortgage loans on real estate, net allowance	5,056,220	5,049,468
Policy loans	380,018	376,254
Policy loans	380,018	376,254
Financial liabilities
Investment contracts	10,394,516	10,003,990
Investment contracts	10,394,516	10,003,990
Notes payable	159,043	137,963
Notes payable	159,043	$ 137,963
Nonrecurring | Level 3 | Corporate debt securities
Financial assets
Fixed maturity securities, bonds held-to-maturity	4,000
Fair value of bonds held-to-maturity	$ 4,000