Fair Value of Financial Instruments - Quantitative Disclosures Regarding Fair Value Hierarchy Measurements (Detail) - USD ($)	Jun. 30, 2019	Dec. 31, 2018
Financial assets
Fixed maturity securities, bonds held-to-maturity	$ 8,394,157,000	$ 8,211,449,000
Fixed maturity securities, bonds held-to-maturity	8,668,599,000	8,130,084,000
Fixed maturity securities, bonds available-for-sale	6,842,917,000	6,215,563,000
Equity securities	1,672,394,000	1,530,228,000
Mortgage loans on real estate, net allowance	5,036,132,000	5,146,040,000
Policy loans	377,669,000	376,254,000
Short-term investments	597,183,000	206,760,000
Separate account assets	1,028,961,000	918,369,000
Total assets	28,386,180,000	26,912,353,000
Financial liabilities
Notes payable	159,577,000	137,963,000
Separate account liabilities	1,028,961,000	918,369,000
Total liabilities	22,618,598,000	21,640,838,000
U.S. states and political subdivisions
Financial assets
Fixed maturity securities, bonds held-to-maturity	207,563,000	245,360,000
Fixed maturity securities, bonds held-to-maturity	214,657,000	250,899,000
Fixed maturity securities, bonds available-for-sale	1,106,031,000	862,030,000
Foreign governments
Financial assets
Fixed maturity securities, bonds held-to-maturity	3,934,000	3,961,000
Fixed maturity securities, bonds held-to-maturity	4,426,000	4,430,000
Fixed maturity securities, bonds available-for-sale	6,361,000	6,210,000
Corporate debt securities
Financial assets
Fixed maturity securities, bonds held-to-maturity	7,849,211,000	7,640,891,000
Fixed maturity securities, bonds held-to-maturity	8,105,793,000	7,548,829,000
Fixed maturity securities, bonds available-for-sale	5,666,546,000	5,283,818,000
Residential mortgage-backed securities
Financial assets
Fixed maturity securities, bonds held-to-maturity	205,408,000	315,306,000
Fixed maturity securities, bonds held-to-maturity	212,780,000	319,910,000
Fixed maturity securities, bonds available-for-sale	24,685,000	31,662,000
Collateralized debt securities
Financial assets
Fixed maturity securities, bonds held-to-maturity	127,892,000	5,214,000
Fixed maturity securities, bonds held-to-maturity	130,793,000	5,285,000
Fixed maturity securities, bonds available-for-sale	10,549,000	3,444,000
Other debt securities
Financial assets
Fixed maturity securities, bonds held-to-maturity	149,000	717,000
Fixed maturity securities, bonds held-to-maturity	150,000	731,000
U.S. treasury and government
Financial assets
Fixed maturity securities, bonds available-for-sale	28,745,000	28,399,000
Recurring
Financial assets
Fixed maturity securities, bonds available-for-sale	6,842,917,000	6,215,563,000
Equity securities	1,672,394,000	1,530,228,000
Equity-indexed options	231,044,000	148,006,000
Short-term investments	597,183,000	206,760,000
Separate account assets	1,012,462,000	905,824,000
Total financial assets	10,356,000,000	9,006,381,000
Financial liabilities
Embedded derivative liability for equity-indexed contracts	695,676,000	596,075,000
Separate account liabilities	1,012,462,000	905,824,000
Total financial liabilities	1,708,138,000	1,501,899,000
Recurring | Common stock
Financial assets
Equity securities	1,651,398,000	1,509,186,000
Recurring | Preferred stock
Financial assets
Equity securities	20,996,000	21,042,000
Recurring | U.S. states and political subdivisions
Financial assets
Fixed maturity securities, bonds available-for-sale	1,106,031,000	862,030,000
Recurring | Foreign governments
Financial assets
Fixed maturity securities, bonds available-for-sale	6,361,000	6,210,000
Recurring | Corporate debt securities
Financial assets
Fixed maturity securities, bonds available-for-sale	5,666,546,000	5,283,818,000
Recurring | Residential mortgage-backed securities
Financial assets
Fixed maturity securities, bonds available-for-sale	24,685,000	31,662,000
Recurring | Collateralized debt securities
Financial assets
Fixed maturity securities, bonds available-for-sale	10,549,000	3,444,000
Recurring | U.S. treasury and government
Financial assets
Fixed maturity securities, bonds available-for-sale	28,745,000	28,399,000
Recurring | Level 1
Financial assets
Fixed maturity securities, bonds available-for-sale	0	0
Equity securities	1,672,281,000	1,530,115,000
Equity-indexed options	0	0
Short-term investments	0	0
Separate account assets	253,053,000	227,448,000
Total financial assets	1,925,334,000	1,757,563,000
Financial liabilities
Embedded derivative liability for equity-indexed contracts	0	0
Separate account liabilities	253,053,000	227,448,000
Total financial liabilities	253,053,000	227,448,000
Recurring | Level 1 | Common stock
Financial assets
Equity securities	1,651,285,000	1,509,073,000
Recurring | Level 1 | Preferred stock
Financial assets
Equity securities	20,996,000	21,042,000
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 1 | U.S. treasury and government
Financial assets
Fixed maturity securities, bonds available-for-sale	0	0
Recurring | Level 2
Financial assets
Fixed maturity securities, bonds available-for-sale	6,838,684,000	6,211,330,000
Equity securities	0	0
Equity-indexed options	0	0
Short-term investments	597,183,000	206,760,000
Separate account assets	759,409,000	678,376,000
Total financial assets	8,195,276,000	7,096,466,000
Financial liabilities
Embedded derivative liability for equity-indexed contracts	0	0
Separate account liabilities	759,409,000	678,376,000
Total financial liabilities	759,409,000	678,376,000
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. states and political subdivisions
Financial assets
Fixed maturity securities, bonds available-for-sale	1,106,031,000	862,030,000
Recurring | Level 2 | Foreign governments
Financial assets
Fixed maturity securities, bonds available-for-sale	6,361,000	6,210,000
Recurring | Level 2 | Corporate debt securities
Financial assets
Fixed maturity securities, bonds available-for-sale	5,662,313,000	5,279,585,000
Recurring | Level 2 | Residential mortgage-backed securities
Financial assets
Fixed maturity securities, bonds available-for-sale	24,685,000	31,662,000
Recurring | Level 2 | Collateralized debt securities
Financial assets
Fixed maturity securities, bonds available-for-sale	10,549,000	3,444,000
Recurring | Level 2 | U.S. treasury and government
Financial assets
Fixed maturity securities, bonds available-for-sale	28,745,000	28,399,000
Recurring | Level 3
Financial assets
Fixed maturity securities, bonds available-for-sale	4,233,000	4,233,000
Equity securities	113,000	113,000
Equity-indexed options	231,044,000	148,006,000
Short-term investments	0	0
Separate account assets	0	0
Total financial assets	235,390,000	152,352,000
Financial liabilities
Embedded derivative liability for equity-indexed contracts	695,676,000	596,075,000
Separate account liabilities	0	0
Total financial liabilities	695,676,000	596,075,000
Recurring | Level 3 | Common stock
Financial assets
Equity securities	113,000	113,000
Recurring | Level 3 | Preferred stock
Financial assets
Equity securities	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	4,233,000	4,233,000
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
Recurring | Level 3 | U.S. treasury and government
Financial assets
Fixed maturity securities, bonds available-for-sale	0	0
Nonrecurring
Financial assets
Fixed maturity securities, bonds held-to-maturity	8,394,157	8,211,449
Fixed maturity securities, bonds held-to-maturity	8,668,599	8,130,084
Total assets	13,817,191	13,728,942
Total financial assets	14,204,518	13,572,338
Financial liabilities
Total liabilities	10,603,781	10,141,953
Total financial liabilities	10,603,781	10,141,953
Nonrecurring | Level 2 | U.S. states and political subdivisions
Financial assets
Fixed maturity securities, bonds held-to-maturity	207,563	245,360
Fixed maturity securities, bonds held-to-maturity	214,657	250,899
Nonrecurring | Level 2 | Foreign governments
Financial assets
Fixed maturity securities, bonds held-to-maturity	3,934	3,961
Fixed maturity securities, bonds held-to-maturity	4,426	4,430
Nonrecurring | Level 2 | Corporate debt securities
Financial assets
Fixed maturity securities, bonds held-to-maturity		7,640,891
Fixed maturity securities, bonds held-to-maturity		7,548,829
Nonrecurring | Level 2 | Residential mortgage-backed securities
Financial assets
Fixed maturity securities, bonds held-to-maturity	205,408	315,306
Fixed maturity securities, bonds held-to-maturity	212,780	319,910
Nonrecurring | Level 2 | Collateralized debt securities
Financial assets
Fixed maturity securities, bonds held-to-maturity	127,892	5,214
Fixed maturity securities, bonds held-to-maturity	130,793	5,285
Nonrecurring | Level 2 | Other debt securities
Financial assets
Fixed maturity securities, bonds held-to-maturity	149	717
Fixed maturity securities, bonds held-to-maturity	150	731
Nonrecurring | Level 3
Financial assets
Mortgage loans on real estate, net allowance	5,014,710	5,124,707
Mortgage loans on real estate, net allowance	5,127,595	5,049,468
Policy loans	377,669	376,254
Policy loans	377,669	376,254
Separately managed accounts	30,655	16,532
Separately managed accounts	30,655	16,532
Financial liabilities
Investment contracts	10,444,204	10,003,990
Investment contracts	10,444,204	10,003,990
Notes payable	159,577	137,963
Notes payable	159,577	$ 137,963
Nonrecurring | Level 2 and 3 | Corporate debt securities
Financial assets
Fixed maturity securities, bonds held-to-maturity	7,849,211
Fixed maturity securities, bonds held-to-maturity	$ 8,105,793