Affiliated Purchasing and Selling of Investments Between Affiliated Companies (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Related Party Transaction [Line Items]
Investments purchased from related parties	$ 445,747	$ 593,984	$ 475,044
Investments purchased from related parties, Limited partnerships	3,673	11,562
Investments purchased from related parties, Derivatives	123,848	110,962	135,918
Investments sold to related parties	211,613	494,524	226,843
Investments sold to related parties, Derivatives	55,289	28,797	76,494
Affiliated companies
Related Party Transaction [Line Items]
Investments purchased from related parties, Limited partnerships		10,000
Investments purchased from related parties, Derivatives	147
Investments sold to related parties, Derivatives	4,440		82
Affiliated companies | Fixed maturity AFS securities
Related Party Transaction [Line Items]
Investments purchased from related parties, Fixed maturities	141,700		30,356
Investments sold to related parties	20,962	$ 89,820	24,878
Affiliated companies | Mortgage loans on real estate
Related Party Transaction [Line Items]
Investments purchased from related parties			$ 4,000
Investments sold to related parties	$ 122,075