Change in Carrying Amount of Value of Business Acquired (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Business Acquisition Acquisition Related Costs [Line Items]
Balance at beginning of period	$ 222,299
Adjustment related to realized (gains) losses on investments and OTTI	8,404	$ 458	$ 1,690
Adjustment related to unrealized (gains) losses and OTTI on investments	4,139	4,819	(14,131)
Balance at end of period	201,299	222,299
Valuation of business acquired
Business Acquisition Acquisition Related Costs [Line Items]
Balance at beginning of period	222,299	259,493
Accretion (amortization) expense	(7,460)	(19,571)
Unlocking	(8,459)	(12,909)
Adjustment related to realized (gains) losses on investments and OTTI	(942)	105	(353)
Adjustment related to unrealized (gains) losses and OTTI on investments	(4,139)	(4,819)
Balance at end of period	$ 201,299	$ 222,299	$ 259,493