Accumulated Other Comprehensive Income (Loss) - Components of Other Comprehensive Income (Loss) (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Balance at beginning of the period	$ 5,255,771	$ 4,661,495
Foreign currency adjustment, net of tax expense (benefit)	(900)	746	$ 289
Balance at end of the period	5,271,515	5,255,771	4,661,495
Net Unrealized Gains (Losses) on Securities
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Balance at beginning of the period	720,911	551,171	457,467
Amounts reclassified from AOCI, net of tax expense (benefit)	(2,111)	(34,895)	(14,308)
Unrealized holding gains arising during the period, net of tax expense (benefit)	(183,981)	210,595	133,451
Unrealized adjustment to DAC, net of tax expense (benefit)	41,017	(1,354)	(18,756)
Unrealized gains on investments attributable to participating policyholders' interest, net of tax expense (benefit)	8,814	(4,606)	(6,683)
Cumulative effect of changes in accounting	(627,119)
Balance at end of the period	(42,469)	720,911	551,171
Defined Benefit Pension Plan Adjustments
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Balance at beginning of the period	(76,562)	(92,393)	(101,679)
Amounts reclassified from AOCI, net of tax expense (benefit)	5,764	18,827	8,242
Actuarial gain (loss) arising during the period net of tax expense (benefit)	16,562	(2,996)	1,044
Balance at end of the period	(54,236)	(76,562)	(92,393)
Foreign Currency Adjustments [Member]
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Balance at beginning of the period	(2,133)	(2,879)	(3,168)
Foreign currency adjustment, net of tax expense (benefit)	(900)	746	289
Balance at end of the period	(3,033)	(2,133)	(2,879)
Accumulated Other Comprehensive Income
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Balance at beginning of the period	642,216	455,899	352,620
Amounts reclassified from AOCI, net of tax expense (benefit)	3,653	(16,068)	(6,066)
Unrealized holding gains arising during the period, net of tax expense (benefit)	(183,981)	210,595	133,451
Unrealized adjustment to DAC, net of tax expense (benefit)	41,017	(1,354)	(18,756)
Unrealized gains on investments attributable to participating policyholders' interest, net of tax expense (benefit)	8,814	(4,606)	(6,683)
Actuarial gain (loss) arising during the period net of tax expense (benefit)	16,562	(2,996)	1,044
Foreign currency adjustment, net of tax expense (benefit)	(900)	746	289
Cumulative effect of changes in accounting	(627,119)
Balance at end of the period	(99,738)	642,216	455,899
Amounts reclassified from AOCI, sale of securities tax expense (benefit)	561	18,789	7,705
Amounts reclassified from AOCI, pension tax expense (benefit)	1,532	5,005	4,438
Unrealized holding gains arising during the period, tax expense (benefit)	46,812	113,604	71,859
Unrealized adjustment to deferred policy acquisition costs, tax expense (benefit)	10,903	729	10,318
Unrealized gains on investments attributable to participating policyholders' interest, tax expense (benefit)	2,343	2,480	3,599
Actuarial gain (loss) arising during the period, tax expense (benefit)	4,402	796	562
Foreign currency adjustment, tax expense (benefit)	239	$ 198	$ 156
Cumulative effect of changes in accounting	$ 334,955