Fair Value of Financial Instruments - Carrying Amount and Fair Value of Financial Instruments (Detail) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
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
Fixed maturity securities, bonds available-for-sale	6,842,917	6,215,563
Equity securities	1,672,394	1,530,228
Mortgage loans on real estate, net of allowance	5,014,710	5,124,707
Policy loans	377,669	376,254
Short-term investments	597,183	206,760
Separate account assets	1,028,961	918,369
Financial liabilities
Notes payable	159,577	137,963
Separate account liabilities	1,028,961	918,369
Fair Value, Inputs, Level 1, 2 and 3
Financial assets
Separate account assets	1,012,462	905,824
Financial liabilities
Separate account liabilities	1,012,462	905,824
Carrying Amount
Financial assets
Fixed maturity securities, bonds held-to-maturity	8,394,157	8,211,449
Fixed maturity securities, bonds available-for-sale	6,842,917	6,215,563
Equity securities	1,672,394	1,530,228
Equity-indexed options	231,044	148,006
Mortgage loans on real estate, net of allowance	5,014,710	5,124,707
Policy loans	377,669	376,254
Short-term investments	597,183	206,760
Separate account assets	1,028,961	918,369
Separately managed accounts	30,655	16,532
Total financial assets	24,189,690	22,747,868
Financial liabilities
Investment contracts	10,444,204	10,003,990
Embedded derivative liability for equity-indexed contracts	695,676	596,075
Notes payable	159,577	137,963
Separate account liabilities	1,028,961	918,369
Total financial liabilities	12,328,418	11,656,397
Fair Value
Financial assets
Fixed maturity securities, bonds held-to-maturity	8,668,599	8,130,084
Fixed maturity securities, bonds available-for-sale	6,842,917	6,215,563
Equity securities	1,672,394	1,530,228
Equity-indexed options	231,044	148,006
Mortgage loans on real estate, net of allowance	5,127,595	5,049,468
Policy loans	377,669	376,254
Short-term investments	597,183	206,760
Separate account assets	1,028,961	918,369
Separately managed accounts	30,655	16,532
Total financial assets	24,577,017	22,591,264
Financial liabilities
Investment contracts	10,444,204	10,003,990
Embedded derivative liability for equity-indexed contracts	695,676	596,075
Notes payable	159,577	137,963
Separate account liabilities	1,028,961	918,369
Total financial liabilities	$ 12,328,418	$ 11,656,397