Business Segments - Assets by Segment (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Segment Reporting, Asset Reconciling Item [Line Items]
Cash and invested assets	$ 17,853,047	$ 15,955,891
Accrued investment income	233,453	223,148
Deferred acquisition costs	3,958,063	3,783,158	$ 3,617,135	$ 3,457,397
Goodwill	441,591	441,591
Other assets	988,831	1,032,299
Total assets	23,474,985	21,436,087
Life
Segment Reporting, Asset Reconciling Item [Line Items]
Deferred acquisition costs	3,423,296	3,261,220
Goodwill	309,609	309,609
Total assets	3,732,905	3,570,829
Health
Segment Reporting, Asset Reconciling Item [Line Items]
Deferred acquisition costs	529,068	512,701
Goodwill	131,982	131,982
Total assets	661,050	644,683
Annuity
Segment Reporting, Asset Reconciling Item [Line Items]
Deferred acquisition costs	5,699	9,237
Total assets	5,699	9,237
Investment
Segment Reporting, Asset Reconciling Item [Line Items]
Cash and invested assets	17,853,047	15,955,891
Accrued investment income	233,453	223,148
Total assets	18,086,500	16,179,039
Other
Segment Reporting, Asset Reconciling Item [Line Items]
Other assets	988,831	1,032,299
Total assets	$ 988,831	$ 1,032,299