Accumulated Other Comprehensive Income (Loss) - Components of Other Comprehensive Income (Loss) (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Balance at beginning of the period	$ 5,271,515	$ 5,255,771	$ 4,661,495
Foreign currency adjustment, net of tax expense (benefit)	390	(900)	746
Balance at end of the period	5,995,763	5,271,515	5,255,771
Net Unrealized Gains (Losses) on Securities
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Balance at beginning of the period	(42,469)	720,911	551,171
Amounts reclassified from AOCI, net of tax expense (benefit)	(800)	(2,111)	(34,895)
Unrealized holding gains arising during the period, net of tax expense (benefit)	266,373	(183,981)	210,595
Unrealized adjustment to DAC, net of tax expense (benefit)	(68,733)	41,017	(1,354)
Unrealized gains on investments attributable to participating policyholders' interest, net of tax expense (benefit)	(12,684)	8,814	(4,606)
Cumulative effect of changes in accounting	16,164	(627,119)
Balance at end of the period	157,851	(42,469)	720,911
Defined Benefit Pension Plan Adjustments
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Balance at beginning of the period	(54,236)	(76,562)	(92,393)
Amounts reclassified from AOCI, net of tax expense (benefit)	5,607	5,764	18,827
Actuarial gain (loss) arising during the period net of tax expense (benefit)	9,888	16,562	(2,996)
Cumulative effect of changes in accounting	(16,491)
Balance at end of the period	(55,232)	(54,236)	(76,562)
Foreign Currency Adjustments
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Balance at beginning of the period	(3,033)	(2,133)	(2,879)
Foreign currency adjustment, net of tax expense (benefit)	390	(900)	746
Cumulative effect of changes in accounting	(458)
Balance at end of the period	(3,101)	(3,033)	(2,133)
Accumulated Other Comprehensive Income (Loss)
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Balance at beginning of the period	(99,738)	642,216	455,899
Amounts reclassified from AOCI, net of tax expense (benefit)	4,807	3,653	(16,068)
Unrealized holding gains arising during the period, net of tax expense (benefit)	266,373	(183,981)	210,595
Unrealized adjustment to DAC, net of tax expense (benefit)	(68,733)	41,017	(1,354)
Unrealized gains on investments attributable to participating policyholders' interest, net of tax expense (benefit)	(12,684)	8,814	(4,606)
Actuarial gain (loss) arising during the period net of tax expense (benefit)	9,888	16,562	(2,996)
Foreign currency adjustment, net of tax expense (benefit)	390	(900)	746
Cumulative effect of changes in accounting	(785)	(627,119)
Balance at end of the period	99,518	(99,738)	642,216
Amounts reclassified from AOCI, sale of securities tax expense (benefit)	213	561	18,789
Amounts reclassified from AOCI, pension tax expense (benefit)	1,491	1,532	5,005
Unrealized holding gains arising during the period, tax expense (benefit)	70,808	46,812	113,604
Unrealized adjustment to deferred policy acquisition costs, tax expense (benefit)	18,270	10,903	729
Unrealized gains on investments attributable to participating policyholders' interest, tax expense (benefit)	3,372	2,343	2,480
Actuarial gain (loss) arising during the period, tax expense (benefit)	2,629	4,402	796
Foreign currency adjustment, tax expense (benefit)	$ 104	239	$ 198
Cumulative effect of changes in accounting		$ 334,955