Accumulated Other Comprehensive Income by Component (Detail) - USD ($) $ in Thousands		3 Months Ended		9 Months Ended		12 Months Ended
		Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance at beginning of period				$ 1,043,441
Total other comprehensive income (loss), net of taxes		$ (2,939)	$ (521)	(59,823)	$ 23,868
Balance at end of period		816,622		816,622		$ 1,043,441
Unrealized Holding Gains (Losses) on AFS Securities and Cash Equivalents
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance at beginning of period		60,272		133,532
Cumulative effect adjustment	[1]			(3,023)
Adjusted balance at beginning of period				130,509
OCI before reclassifications		(8,277)		(66,154)
Gain (loss) amounts reclassified from AOCI		(1,212)		(13,572)
Net current period OCI		(9,489)		(79,726)
Tax expense (benefit)		0		0
Total other comprehensive income (loss), net of taxes		(9,489)		(79,726)
Balance at end of period		50,783		50,783		133,532
Unrealized Holding Gains (Losses) on Cash Flow Hedges
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance at beginning of period		(650)	2,086	(931)	(1,225)	(1,225)
Adjusted balance at beginning of period				(931)
OCI before reclassifications		66	(1,803)	(1,050)	888
Gain (loss) amounts reclassified from AOCI		165	63	1,562	683
Net current period OCI		231	(1,740)	512	1,571
Tax expense (benefit)		0	0	0	0
Total other comprehensive income (loss), net of taxes		231	(1,740)	512	1,571
Balance at end of period		(419)	346	(419)	346	(931)
OCI Adjustments for Policy holder liabilities, VOBA, and Deferred Tax
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance at beginning of period		(39,030)	(51,922)	(52,102)	(47,963)	(47,963)
Adjusted balance at beginning of period				(52,102)
OCI before reclassifications		6,319	(780)	19,391	(4,739)
Net current period OCI		6,319	(780)	19,391	(4,739)
Tax expense (benefit)		0	0	0	0
Total other comprehensive income (loss), net of taxes		6,319	(780)	19,391	(4,739)
Balance at end of period		(32,711)	(52,702)	(32,711)	(52,702)	(52,102)
Accumulated Other Comprehensive Income (Loss)
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance at beginning of period		20,592	79,739	80,499	55,350	55,350
Cumulative effect adjustment	[1],[2]			(3,023)
Adjusted balance at beginning of period				77,476	55,350	55,350
OCI before reclassifications		(1,892)	(584)	(47,813)	23,264
Gain (loss) amounts reclassified from AOCI		(1,047)	63	(12,010)	604
Net current period OCI		(2,939)	(521)	(59,823)	23,868
Tax expense (benefit)		0	0	0	0
Total other comprehensive income (loss), net of taxes		(2,939)	(521)	(59,823)	23,868	25,149
Balance at end of period		$ 17,653	79,218	$ 17,653	79,218	80,499
Unrealized Holding Gains (Losses) on AFS Securities
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance at beginning of period			129,575		104,538	$ 104,538
OCI before reclassifications			1,999		27,115
Gain (loss) amounts reclassified from AOCI					(79)
Net current period OCI			1,999		27,036
Tax expense (benefit)			0		0
Total other comprehensive income (loss), net of taxes			1,999		27,036
Balance at end of period			$ 131,574		$ 131,574

[1] See Notes to Financial Statements – Note 1, Summary of Significant Accounting Policies, Current Accounting Guidance, for discussion on the adjustment relating to ASU 2016-01.

[2] See Notes to Financial Statements – Note 1, Summary of Significant Accounting Policies, Current Accounting Guidance, for discussion on the adjustment relating to Accounting Standards Update (“ASU”) 2016-01.