Segment Information - Summary of Total Assets by Operating Segments (Detail) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Segment Reporting Information [Line Items]
Total Assets	$ 26,912,353	$ 26,386,764	$ 24,533,222
Life
Segment Reporting Information [Line Items]
Total Assets	6,263,366	6,101,458	5,921,208
Annuity
Segment Reporting Information [Line Items]
Total Assets	12,900,650	12,345,215	11,310,936
Health
Segment Reporting Information [Line Items]
Total Assets	527,525	468,947	472,369
Property & Casualty
Segment Reporting Information [Line Items]
Total Assets	2,216,628	2,189,515	2,046,303
Corporate and Other
Segment Reporting Information [Line Items]
Total Assets	$ 5,004,184	$ 5,281,629	$ 4,782,406