Schedule III - Supplementary Insurance Information (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
SEC Schedule, 12-16, Insurance Companies, Supplementary Insurance Information [Line Items]
Deferred Policy Acquisition Cost	$ 1,497,261	$ 1,373,844	$ 1,294,443
Future Policy Benefits, Policyholders’ Account Balances, Policy and Contract Claims and Other Policyholder Funds	18,884,849	18,239,714	16,968,156
Unearned Premiums	908,856	875,294	823,938
Premium Revenue	2,228,193	2,067,202	1,996,648
Net Investment Income	858,367	966,077	860,235
Benefits, Claims, Losses and Settlement Expenses	1,879,972	1,718,203	1,742,008
Amortization of Deferred Policy Acquisition Costs	480,157	444,351	461,365
Other Operating Expenses	497,011	485,340	476,462
Premiums Written	1,514,563	1,414,024	1,282,876
Life
SEC Schedule, 12-16, Insurance Companies, Supplementary Insurance Information [Line Items]
Deferred Policy Acquisition Cost	839,133	791,276	745,840
Future Policy Benefits, Policyholders’ Account Balances, Policy and Contract Claims and Other Policyholder Funds	5,158,377	5,432,688	4,937,467
Unearned Premiums	29,901	33,298	35,133
Premium Revenue	350,012	328,570	318,953
Net Investment Income	233,181	245,835	227,923
Benefits, Claims, Losses and Settlement Expenses	417,702	410,152	429,813
Amortization of Deferred Policy Acquisition Costs	97,263	74,068	112,712
Other Operating Expenses	190,835	190,482	186,879
Annuity
SEC Schedule, 12-16, Insurance Companies, Supplementary Insurance Information [Line Items]
Deferred Policy Acquisition Cost	499,588	426,497	394,208
Future Policy Benefits, Policyholders’ Account Balances, Policy and Contract Claims and Other Policyholder Funds	12,372,418	11,533,813	10,821,889
Premium Revenue	231,027	222,207	248,714
Net Investment Income	467,788	573,789	500,726
Benefits, Claims, Losses and Settlement Expenses	290,611	270,970	296,586
Amortization of Deferred Policy Acquisition Costs	57,468	74,750	71,381
Other Operating Expenses	46,859	44,486	51,283
Health
SEC Schedule, 12-16, Insurance Companies, Supplementary Insurance Information [Line Items]
Deferred Policy Acquisition Cost	33,960	36,806	40,620
Future Policy Benefits, Policyholders’ Account Balances, Policy and Contract Claims and Other Policyholder Funds	319,789	282,872	272,802
Unearned Premiums	37,261	40,665	43,155
Premium Revenue	180,414	156,436	175,589
Net Investment Income	9,376	9,538	9,942
Benefits, Claims, Losses and Settlement Expenses	122,547	103,037	132,390
Amortization of Deferred Policy Acquisition Costs	15,436	15,227	14,973
Other Operating Expenses	41,819	38,475	42,655
Property and Casualty
SEC Schedule, 12-16, Insurance Companies, Supplementary Insurance Information [Line Items]
Deferred Policy Acquisition Cost	124,580	119,265	113,775
Future Policy Benefits, Policyholders’ Account Balances, Policy and Contract Claims and Other Policyholder Funds	1,034,265	990,341	935,998
Unearned Premiums	841,694	801,331	745,650
Premium Revenue	1,466,740	1,359,989	1,253,392
Net Investment Income	62,320	61,688	57,091
Benefits, Claims, Losses and Settlement Expenses	1,049,112	934,044	883,219
Amortization of Deferred Policy Acquisition Costs	309,990	280,306	262,299
Other Operating Expenses	186,019	177,345	165,278
Premiums Written	1,514,563	1,414,024	1,282,876
Corporate and Other
SEC Schedule, 12-16, Insurance Companies, Supplementary Insurance Information [Line Items]
Net Investment Income	85,702	75,227	64,553
Other Operating Expenses	$ 31,479	$ 34,552	$ 30,367