Fair Value of Financial Instruments - Carrying Amount and Fair Value of Financial Instruments (Detail) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Financial assets
Fair value of bonds held-to-maturity	$ 9,063,688	$ 8,130,084
Fixed maturity securities, bonds available-for-sale	6,826,245	6,215,563
Equity securities	1,678,657	1,530,228
Policy loans	380,018	376,254
Short-term investments	398,948	206,760
Separate account assets	1,025,370	918,369
Financial liabilities
Separate account liabilities	1,025,370	918,369
Fair Value, Inputs, Level 1, 2 and 3
Financial assets
Separate account assets	1,008,309	905,824
Financial liabilities
Separate account liabilities	1,008,309	905,824
Carrying Amount
Financial assets
Fair value of bonds held-to-maturity	8,715,569	8,211,449
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
Mortgage loans on real estate, net allowance	4,936,605	5,124,707
Policy loans	380,018	376,254
Short-term investments	398,948	206,760
Separate account assets	1,025,370	918,369
Separately managed accounts	40,093	16,532
Total financial assets	24,225,689	22,747,868
Financial liabilities
Investment contracts	10,394,516	10,003,990
Embedded derivative liability for equity-indexed contracts	691,766	596,075
Notes payable	159,043	137,963
Separate account liabilities	1,025,370	918,369
Total financial liabilities	12,270,695	11,656,397
Fair Value
Financial assets
Fair value of bonds held-to-maturity	9,063,688	8,130,084
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
Mortgage loans on real estate, net allowance	5,056,220	5,049,468
Policy loans	380,018	376,254
Short-term investments	398,948	206,760
Separate account assets	1,025,370	918,369
Separately managed accounts	40,093	16,532
Total financial assets	24,693,423	22,591,264
Financial liabilities
Investment contracts	10,394,516	10,003,990
Embedded derivative liability for equity-indexed contracts	691,766	596,075
Notes payable	159,043	137,963
Separate account liabilities	1,025,370	918,369
Total financial liabilities	$ 12,270,695	$ 11,656,397

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