Accumulated Other Comprehensive Income (Loss) - Components of Other Comprehensive Income (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Balance at beginning of the period	$ 5,995,763	$ 5,271,515
Foreign currency adjustment, net of tax benefit	(924)	(156)
Balance at end of the period	5,607,089	5,592,919
Net Unrealized Gains (Losses) on Securities
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Balance at beginning of the period	157,851	(42,469)
Amounts reclassified from AOCI, net of tax expense (benefit)	9,606	(1,843)
Unrealized holding gains (losses) arising during the period, net of tax expense (benefit)	(201,385)	124,158
Unrealized adjustment to DAC, net of tax expense (benefit)	70,567	(30,726)
Unrealized gains (losses) on investments attributable to participating policyholders' interest, net of tax expense (benefit)	8,809	(6,075)
Cumulative effect of changes in accounting		16,166
Balance at end of the period	45,448	59,211
Defined Benefit Pension Plan Adjustments
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Balance at beginning of the period	(55,232)	(54,236)
Amounts reclassified from AOCI, net of tax expense (benefit)	1,756	1,416
Cumulative effect of changes in accounting		(16,493)
Balance at end of the period	(53,476)	(69,313)
Foreign Currency Adjustments
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Balance at beginning of the period	(3,101)	(3,033)
Foreign currency adjustment, net of tax benefit	(924)	(156)
Cumulative effect of changes in accounting		(458)
Balance at end of the period	(4,025)	(3,647)
Accumulated Other Comprehensive Income (Loss)
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Balance at beginning of the period	99,518	(99,738)
Amounts reclassified from AOCI, net of tax expense (benefit)	11,362	(427)
Unrealized holding gains (losses) arising during the period, net of tax expense (benefit)	(201,385)	124,158
Unrealized adjustment to DAC, net of tax expense (benefit)	70,567	(30,726)
Unrealized gains (losses) on investments attributable to participating policyholders' interest, net of tax expense (benefit)	8,809	(6,075)
Foreign currency adjustment, net of tax benefit	(924)	(156)
Cumulative effect of changes in accounting		(785)
Balance at end of the period	(12,053)	(13,749)
Amounts reclassified from AOCI, sale of securities tax expense (benefit)	2,554	490
Amounts reclassified from AOCI, pension tax expense (benefit)	467	376
Unrealized holding gains (losses) arising during the period, tax expense	(53,533)	33,004
Unrealized adjustment to deferred policy acquisition costs, tax expense (benefit)	18,759	8,167
Unrealized gains (losses) on investments attributable to participating policyholders' interest, tax expense (benefit)	(2,342)	1,615
Foreign currency adjustment, tax expense (benefit)	$ 246	$ 41