Accumulated Other Comprehensive Income by Component (Detail) - USD ($) $ in Thousands		3 Months Ended			6 Months Ended			12 Months Ended
		Jun. 30, 2016	Jun. 30, 2015		Jun. 30, 2016	Jun. 30, 2015		Dec. 31, 2015
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance at beginning of period					$ 1,291,501
OCI before reclassifications	[1]	$ 34,758	$ (59,767)		76,378	$ (36,208)
Amounts reclassified from AOCI	[1]	581	686		(1,295)	1,048
Total other comprehensive income (loss), net of taxes	[1]	35,339	(59,081)		75,083	(35,160)
Balance at end of period		1,277,180			1,277,180			$ 1,291,501
Unrealized Holding Gains (Losses) on AFS Securities
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance at beginning of period		141,722	191,016	[1]	95,533	164,221	[1]	164,221	[1]
OCI before reclassifications		40,786	(68,286)	[1]	88,654	(42,653)	[1]
Amounts reclassified from AOCI		68	60	[1]	(1,611)	1,222	[1]
Total other comprehensive income (loss), net of taxes		40,854	(68,226)	[1]	87,043	(41,431)	[1]
Balance at end of period		182,576	122,790	[1]	182,576	122,790	[1]	95,533
Unrealized Holding Gains (Losses) on Cash Flow Hedges
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance at beginning of period		2,855	2,584	[1]	4,202	2,296	[1]	2,296	[1]
OCI before reclassifications		1,290	(362)	[1]	140	726	[1]
Amounts reclassified from AOCI		513	626	[1]	316	(174)	[1]
Total other comprehensive income (loss), net of taxes		1,803	264	[1]	456	552	[1]
Balance at end of period		4,658	2,848	[1]	4,658	2,848	[1]	4,202
OCI Adjustments for Policy holder liabilities, VOBA, and Deferred Tax
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance at beginning of period		(48,242)	(60,437)		(43,144)	(57,275)		(57,275)
OCI before reclassifications		(7,318)	8,881		(12,416)	5,719
Total other comprehensive income (loss), net of taxes		(7,318)	8,881		(12,416)	5,719
Balance at end of period		(55,560)	(51,556)		(55,560)	(51,556)		(43,144)
Accumulated Other Comprehensive Income
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance at beginning of period	[1]	96,335	133,163		56,591	109,242		109,242
Total other comprehensive income (loss), net of taxes					75,083			(52,651)
Balance at end of period	[1]	$ 131,674	$ 74,082		$ 131,674	$ 74,082		$ 56,591

[1] All amounts are net of tax.