Schedule II - Condensed Financial Information of Registrant - Condensed Statements of Cash Flows (Detail) - USD ($)	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
OPERATING ACTIVITIES
Net income	$ 171,092,000	$ 105,943,000	$ 97,875,000	$ 256,867,000	$ (117,739,000)	$ 175,841,000	$ 84,062,000	$ 18,258,000	$ 631,777,000	$ 160,422,000	$ 495,607,000
Adjustments to reconcile net income to net cash provided by operating activities
Net realized investment gains									(37,719,000)	(18,174,000)
Other-than-temporary impairments									6,968,000	1,243,000	13,386,000
Accretion of premiums, discounts and loan origination fees									(4,394,000)	(6,163,000)	(2,008,000)
Net capitalized interest on policy loans and mortgage loans									(34,081,000)	(39,262,000)	(32,551,000)
Depreciation									53,160,000	52,049,000	53,937,000
Interest credited to policyholders’ account balances									511,999,000	315,684,000	415,190,000
Charges to policyholders’ account balances									(305,256,000)	(285,549,000)	(248,526,000)
Deferred federal income tax expense (benefit)									78,385,000	(22,599,000)	(142,096,000)
Changes in
Policyholder liabilities									145,396,000	288,065,000	320,743,000
Deferred policy acquisition costs									(12,749,000)	(71,497,000)	(81,484,000)
Reinsurance recoverables									15,645,000	(8,886,000)	(16,880,000)
Premiums due and other receivables									3,780,000	(31,360,000)	(19,445,000)
Prepaid reinsurance premiums									8,952,000	10,003,000	(599,000)
Accrued investment income									(12,227,000)	(960,000)	(7,347,000)
Current tax receivable/payable									18,893,000	35,315,000	17,252,000
Liability for retirement benefits									(8,454,000)	(69,762,000)	(14,127,000)
Fair value of option securities									(144,978,000)	54,951,000	(90,357,000)
Fair value of equity securities									(422,535,000)	107,188,000	0
Other, net									5,503,000	24,630,000	4,958,000
Net cash provided by operating activities									506,412,000	495,510,000	495,925,000
Proceeds from sale/maturity/prepayment of
Held-to-maturity securities									864,481,000	629,359,000	893,977,000
Available-for-sale securities									500,724,000	451,292,000	489,902,000
Equity securities									294,798,000	214,737,000	137,780,000
Investment real estate									66,725,000	11,577,000	67,227,000
Mortgage loans									837,732,000	812,239,000	811,049,000
Policy loans									48,079,000	52,606,000	42,580,000
Other invested assets									120,455,000	118,846,000	80,901,000
Disposals of property and equipment									69,000	0	554,000
Distributions from affiliates and subsidiaries									91,325,000	58,287,000	102,000,000
Payment for the purchase/origination of
Held-to-maturity securities									(1,468,253,000)	(1,349,008,000)	(1,215,311,000)
Available-for-sale securities									(528,495,000)	(680,477,000)
Available-for-sale securities											(737,651,000)
Equity securities									(49,016,000)	(79,514,000)	(108,054,000)
Investment real estate									(24,163,000)	(71,732,000)	(33,844,000)
Mortgage loans									(784,408,000)	(1,173,189,000)	(1,194,672,000)
Policy loans									(27,722,000)	(26,147,000)	(23,325,000)
Other invested assets									(109,074,000)	(75,233,000)	(47,134,000)
Additions to property and equipment									(21,402,000)	(17,670,000)	(24,395,000)
Contributions to unconsolidated affiliates									(262,569,000)	(151,261,000)	(27,869,000)
Change in short-term investments									(190,511,000)	452,005,000	(466,539,000)
Change in collateral held for derivatives									107,133,000	(68,565,000)	89,981,000
Other, net									10,369,000	7,087,000	18,030,000
Net cash used in investing activities									(523,723,000)	(884,761,000)	(1,144,813,000)
FINANCING ACTIVITIES
Policyholders’ account deposits									1,770,646,000	1,717,153,000	2,095,734,000
Policyholders’ account withdrawals									(1,481,234,000)	(1,345,498,000)	(1,271,128,000)
Dividends to stockholders									(88,243,000)	(88,228,000)	(88,335,000)
Net cash provided by financing activities									201,148,000	281,578,000	735,387,000
Beginning of the year				268,164,000					268,164,000
End of the year	452,001,000				268,164,000				452,001,000	268,164,000
Parent Company
OPERATING ACTIVITIES
Net income									620,363,000	158,995,000	493,651,000
Adjustments to reconcile net income to net cash provided by operating activities
Net realized investment gains									(5,600,000)	(2,053,000)	(21,052,000)
Other-than-temporary impairments									6,663,000	1,243,000	6,105,000
Accretion of premiums, discounts and loan origination fees									(9,654,000)	(11,236,000)	(6,615,000)
Net capitalized interest on policy loans and mortgage loans									(31,366,000)	(36,784,000)	(31,853,000)
Depreciation									30,658,000	30,492,000	32,991,000
Interest credited to policyholders’ account balances									441,268,000	269,933,000	370,270,000
Charges to policyholders’ account balances									(291,160,000)	(272,638,000)	(236,336,000)
Deferred federal income tax expense (benefit)									40,936,000	10,564,000	(57,337,000)
Equity in earnings of affiliates									(6,752,000)	(8,323,000)	(10,840,000)
Net income of subsidiaries									(486,136,000)	(16,466,000)	(275,739,000)
Distributions from equity method investments									22,012,000	5,319,000	0
Changes in
Policyholder liabilities									44,732,000	165,931,000	179,497,000
Deferred policy acquisition costs									(4,791,000)	(61,881,000)	(66,219,000)
Reinsurance recoverables									18,826,000	(9,855,000)	584,000
Premiums due and other receivables									4,333,000	(1,302,000)	12,343,000
Prepaid reinsurance premiums									5,277,000	3,213,000	3,392,000
Accrued investment income									(3,535,000)	2,306,000	(3,138,000)
Current tax receivable/payable									(83,470,000)	79,168,000	1,725,000
Liability for retirement benefits									(5,482,000)	(64,824,000)	(31,830,000)
Fair value of option securities									(134,925,000)	50,299,000	(86,259,000)
Fair value of equity securities									(958,000)	208,000	0
Other, net									(6,193,000)	(17,943,000)	20,437,000
Net cash provided by operating activities									165,046,000	274,366,000	293,777,000
Proceeds from sale/maturity/prepayment of
Held-to-maturity securities									706,230,000	514,393,000	730,143,000
Available-for-sale securities									366,696,000	296,545,000	315,030,000
Equity securities									0	0	5,635,000
Investment real estate									0	3,782,000	58,840,000
Mortgage loans									789,088,000	799,413,000	794,011,000
Policy loans									42,316,000	42,407,000	44,253,000
Other invested assets									112,340,000	110,415,000	76,521,000
Disposals of property and equipment									69,000	0	158,000
Distributions from affiliates and subsidiaries									27,891,000	17,795,000	20,038,000
Payment for the purchase/origination of
Held-to-maturity securities									(936,646,000)	(971,396,000)	(824,831,000)
Available-for-sale securities									(326,476,000)	(535,233,000)
Available-for-sale securities											(345,677,000)
Equity securities									(351,000)	(1,485,000)	(128,000)
Investment real estate									(13,639,000)	(23,790,000)	(26,936,000)
Mortgage loans									(668,563,000)	(1,021,303,000)	(1,117,320,000)
Policy loans									(25,408,000)	(23,014,000)	(22,978,000)
Other invested assets									(102,275,000)	(67,914,000)	(42,849,000)
Additions to property and equipment									(11,163,000)	(10,767,000)	(20,441,000)
Contributions to unconsolidated affiliates									(147,547,000)	(95,091,000)	(26,056,000)
Change in short-term investments									138,021,000	360,837,000	(401,110,000)
Change in investment in subsidiaries									35,069,000	100,000,000	0
Change in collateral held for derivatives									97,852,000	(63,069,000)	84,325,000
Other, net									(92,000)	191,000	15,751,000
Net cash used in investing activities									83,412,000	(567,284,000)	(683,621,000)
FINANCING ACTIVITIES
Policyholders’ account deposits									1,375,003,000	1,513,478,000	1,732,494,000
Policyholders’ account withdrawals									(1,391,881,000)	(1,243,641,000)	(1,182,754,000)
Dividends to stockholders									(88,243,000)	(88,228,000)	(88,335,000)
Net cash provided by financing activities									(105,121,000)	181,609,000	461,405,000
NET INCREASE (DECREASE) IN CASH AND CASH EQUIVALENTS									143,337,000	(111,309,000)	71,561,000
Beginning of the year				$ 151,592,000				$ 262,901,000	151,592,000	262,901,000	191,340,000
End of the year	$ 294,929,000				$ 151,592,000				$ 294,929,000	$ 151,592,000	$ 262,901,000