Affiliated Purchasing and Selling of Investments Between Affiliated Companies (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Related Party Transaction [Line Items]
Investments purchased from related parties, Derivatives			$ 57,534	$ 79,808
Investments sold to related parties	$ 98,414	$ 12,264	223,147	65,967
Investments sold to related parties, Derivatives			14,223	48,623
Affiliated companies
Related Party Transaction [Line Items]
Investments purchased from related parties, Fixed maturities		4,490		4,490
Investments purchased from related parties, Derivatives		150		150
Investments sold to related parties, Derivatives		$ 4,440		$ 4,440
Affiliated companies | Fixed maturity AFS securities
Related Party Transaction [Line Items]
Investments sold to related parties			$ 12,832