Accumulated Other Comprehensive Income (Loss) - Components of Other Comprehensive Income (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Balance at beginning of the period	$ 5,592,919	$ 5,221,510	$ 5,271,515	$ 5,255,771
Foreign currency adjustment, net of tax benefit	746	(134)	590	(500)
Balance at end of the period	5,767,582	5,242,067	5,767,582	5,242,067
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)	716,878
Amounts reclassified from AOCI, net of tax expense (benefit)			2,126	(1,740)
Unrealized holding gains (losses) arising during the period, net of tax expense (benefit)			219,402	(168,551)
Unrealized adjustment to DAC, net of tax expense (benefit)			(27,533)	33,150
Unrealized gains (losses) on investments attributable to participating policyholders' interest, net of tax expense (benefit)			(10,121)	9,420
Cumulative effect of changes in accounting	16,166	(627,119)	16,166	(627,119)
Balance at end of the period	157,571	(37,962)	157,571	(37,962)
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)	(72,772)
Amounts reclassified from AOCI, net of tax expense (benefit)			2,911	2,390
Cumulative effect of changes in accounting	(16,493)		(16,493)
Balance at end of the period	(67,818)	(70,382)	(67,818)	(70,382)
Foreign Currency Adjustments
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Balance at beginning of the period			(3,033)	(1,890)
Foreign currency adjustment, net of tax benefit			590	(500)
Cumulative effect of changes in accounting	(458)		(458)
Balance at end of the period	(2,901)	(2,390)	(2,901)	(2,390)
Accumulated Other Comprehensive Income (Loss)
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Balance at beginning of the period	(13,749)	(86,070)	(99,738)	642,216
Amounts reclassified from AOCI, net of tax expense (benefit)			5,037	650
Unrealized holding gains (losses) arising during the period, net of tax expense (benefit)			219,402	(168,551)
Unrealized adjustment to DAC, net of tax expense (benefit)			(27,533)	33,150
Unrealized gains (losses) on investments attributable to participating policyholders' interest, net of tax expense (benefit)			(10,121)	9,420
Foreign currency adjustment, net of tax benefit			590	(500)
Cumulative effect of changes in accounting	(785)	(627,119)	(785)	(627,119)
Balance at end of the period	$ 86,852	$ (110,734)	86,852	(110,734)
Amounts reclassified from AOCI, sale of securities tax expense (benefit)			565	462
Amounts reclassified from AOCI, pension tax expense (benefit)			774	635
Unrealized holding gains (losses) arising during the period, tax expense			58,322	(39,660)
Unrealized adjustment to deferred policy acquisition costs, tax expense (benefit)			7,320	8,812
Unrealized gains (losses) on investments attributable to participating policyholders' interest, tax expense (benefit)			(2,690)	2,504
Foreign currency adjustment, tax expense (benefit)			$ 157	(133)
Cumulative effect of changes in accounting, net of tax benefit				$ 334,955