Accumulated Other Comprehensive Income (Loss) - Components of Other Comprehensive Income (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Balance at beginning of the period	$ 5,767,582	$ 5,242,067	$ 5,271,515	$ 5,255,771
Foreign currency adjustment, net of tax benefit	(293)	(181)	297	(681)
Balance at end of the period	5,831,708	5,381,670	5,831,708	5,381,670
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)			1,510	(2,282)
Unrealized holding gains (losses) arising during the period, net of tax expense (benefit)			256,880	(181,582)
Unrealized adjustment to DAC, net of tax expense (benefit)			(65,244)	30,708
Unrealized gains (losses) on investments attributable to participating policyholders' interest, net of tax expense (benefit)			(11,841)	11,040
Cumulative effect of changes in accounting	16,164	(627,119)	16,164	(627,119)
Balance at end of the period	155,000	(52,357)	155,000	(52,357)
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)			4,365	3,991
Cumulative effect of changes in accounting	(16,491)		(16,491)
Balance at end of the period	(66,362)	(68,781)	(66,362)	(68,781)
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			297	(681)
Cumulative effect of changes in accounting	(458)		(458)
Balance at end of the period	(3,194)	(2,571)	(3,194)	(2,571)
Accumulated Other Comprehensive Income (Loss)
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Balance at beginning of the period	86,852	(110,734)	(99,738)	642,216
Amounts reclassified from AOCI, net of tax expense (benefit)			5,875	1,709
Unrealized holding gains (losses) arising during the period, net of tax expense (benefit)			256,880	(181,582)
Unrealized adjustment to DAC, net of tax expense (benefit)			(65,244)	30,708
Unrealized gains (losses) on investments attributable to participating policyholders' interest, net of tax expense (benefit)			(11,841)	11,040
Foreign currency adjustment, net of tax benefit			297	(681)
Cumulative effect of changes in accounting	(785)	(627,119)	(785)	(627,119)
Balance at end of the period	$ 85,444	$ (123,709)	85,444	(123,709)
Amounts reclassified from AOCI, sale of securities tax expense (benefit)			401	606
Amounts reclassified from AOCI, pension tax expense (benefit)			1,160	1,061
Unrealized holding gains (losses) arising during the period, tax expense			68,285	(47,963)
Unrealized adjustment to deferred policy acquisition costs, tax expense (benefit)			17,343	8,163
Unrealized gains (losses) on investments attributable to participating policyholders' interest, tax expense (benefit)			(3,148)	2,935
Foreign currency adjustment, tax expense (benefit)			$ 79	(181)
Cumulative effect of changes in accounting, net of tax benefit				$ 334,955