Accumulated Other Comprehensive Income (Loss) - Components of Other Comprehensive Income (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Balance at beginning of the period	$ 5,271,515	$ 5,255,771
Foreign currency adjustment, net of tax benefit	(156)	(366)
Balance at end of the period	5,592,919	5,221,510
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
Amounts reclassified from AOCI, net of tax expense (benefit)	(1,843)	100
Unrealized holding losses arising during the period, net of tax expense	124,158	(113,203)
Unrealized adjustment to DAC, net of tax expense (benefit)	(30,726)	16,277
Unrealized losses on investments attributable to participating policyholders' interest, net of tax expense (benefit)	(6,075)	5,493
Cumulative effect of changes in accounting	16,166	(637,376)
Balance at end of the period	59,211	(7,798)
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)
Amounts reclassified from AOCI, net of tax expense (benefit)	1,416	789
Cumulative effect of changes in accounting	(16,493)
Balance at end of the period	(69,313)	(75,773)
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)
Foreign currency adjustment, net of tax benefit	(156)	(366)
Cumulative effect of changes in accounting	(458)
Balance at end of the period	(3,647)	(2,499)
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
Amounts reclassified from AOCI, net of tax expense (benefit)	(427)	889
Unrealized holding losses arising during the period, net of tax expense	124,158	(113,203)
Unrealized adjustment to DAC, net of tax expense (benefit)	(30,726)	16,277
Unrealized losses on investments attributable to participating policyholders' interest, net of tax expense (benefit)	(6,075)	5,493
Foreign currency adjustment, net of tax benefit	(156)	(366)
Cumulative effect of changes in accounting	(785)	(637,376)
Balance at end of the period	(13,749)	(86,070)
Amounts reclassified from AOCI, sale of securities tax expense (benefit)	490	26
Amounts reclassified from AOCI, pension tax expense (benefit)	376	210
Unrealized holding gains (losses) arising during the period, tax expense	33,004	(30,091)
Unrealized adjustment to deferred policy acquisition costs, tax expense (benefit)	(8,167)	3,777
Unrealized gains (losses) on investments attributable to participating policyholders' interest, tax expense (benefit)	(1,615)	1,460
Foreign currency adjustment, tax expense (benefit)	$ (41)	(97)
Cumulative Effect of New Accounting Principle in Period of Adoption, Adjustment Tax		$ 356,847