Affiliated Purchasing and Selling of Investments Between Affiliated Companies (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Related Party Transaction [Line Items]
Investments purchased from related parties, Derivatives			$ 85,560	$ 91,553
Investments sold to related parties	$ 37,569	$ 46,439	260,716	112,406
Investments sold to related parties, Derivatives			38,215	50,677
Affiliated companies
Related Party Transaction [Line Items]
Investments purchased from related parties, Fixed maturities		113,364		117,854
Investments purchased from related parties, Derivatives				150
Investments sold to related parties, Derivatives				4,440
Affiliated companies | Fixed maturity AFS securities
Related Party Transaction [Line Items]
Investments sold to related parties			$ 12,832
Affiliated companies | Mortgages
Related Party Transaction [Line Items]
Investments sold to related parties		$ 122,075		$ 122,075