Components of Net Unrealized Gains Losses and Other-Than-Temporary Impairments Included in Accumulated Other Comprehensive Income Net of Taxes (Detail) - USD ($) $ in Thousands	Jun. 30, 2017	Dec. 31, 2016
Accumulated Other Comprehensive Income (Loss) [Line Items]
Accumulated other comprehensive income (loss), net of taxes	$ 79,739	$ 55,350
Unrealized Holding Gains (Losses) on AFS Securities
Accumulated Other Comprehensive Income (Loss) [Line Items]
Accumulated other comprehensive income (loss), before taxes	101,072	76,683
Unrealized Holding Gains (Losses) on AFS Securities | Fixed maturity AFS securities
Accumulated Other Comprehensive Income (Loss) [Line Items]
Accumulated other comprehensive income (loss), before taxes	126,908	103,250
Unrealized Holding Gains (Losses) on AFS Securities | Equity AFS securities
Accumulated Other Comprehensive Income (Loss) [Line Items]
Accumulated other comprehensive income (loss), before taxes	2,665	1,287
Unrealized Holding Gains (Losses) on AFS Securities | Valuation of business acquired
Accumulated Other Comprehensive Income (Loss) [Line Items]
Accumulated other comprehensive income (loss), before taxes	(30,589)	(26,630)
Unrealized Holding Gains (Losses) on Cash Flow Hedges
Accumulated Other Comprehensive Income (Loss) [Line Items]
Accumulated other comprehensive income (loss), before taxes	2,088	(1,224)
Accumulated Net Unrealized Liability Gain (Loss) | Federal income taxes - deferred
Accumulated Other Comprehensive Income (Loss) [Line Items]
Accumulated other comprehensive income (loss), before taxes	$ (21,333)	$ (21,333)