Accumulated Other Comprehensive Income by Component (Detail) - USD ($) $ in Thousands		3 Months Ended		6 Months Ended		12 Months Ended
		Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 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		$ (10,508)	$ 18,401	(56,884)	$ 24,389
Balance at end of period		806,012		806,012		$ 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		74,860		133,532
Cumulative effect adjustment	[1]			(3,023)
Adjusted balance at beginning of period				130,509
OCI before reclassifications		(13,678)		(57,877)
Gain (loss) amounts reclassified from AOCI		(910)		(12,360)
Net current period OCI		(14,588)		(70,237)
Tax expense (benefit)		0		0
Total other comprehensive income (loss), net of taxes		(14,588)		(70,237)
Balance at end of period		60,272		60,272		133,532
Unrealized Holding Gains (Losses) on Cash Flow Hedges
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance at beginning of period		(442)	(817)	(931)	(1,225)	(1,225)
Adjusted balance at beginning of period				(931)
OCI before reclassifications		(2,908)	2,530	(1,116)	2,691
Gain (loss) amounts reclassified from AOCI		2,700	373	1,397	620
Net current period OCI		(208)	2,903	281	3,311
Tax expense (benefit)		0	0	0	0
Total other comprehensive income (loss), net of taxes		(208)	2,903	281	3,311
Balance at end of period		(650)	2,086	(650)	2,086	(931)
OCI Adjustments for Policy holder liabilities, VOBA, and Deferred Tax
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance at beginning of period		(43,318)	(48,834)	(52,102)	(47,963)	(47,963)
Adjusted balance at beginning of period				(52,102)
OCI before reclassifications		4,288	(3,088)	13,072	(3,959)
Net current period OCI		4,288	(3,088)	13,072	(3,959)
Tax expense (benefit)		0	0	0	0
Total other comprehensive income (loss), net of taxes		4,288	(3,088)	13,072	(3,959)
Balance at end of period		(39,030)	(51,922)	(39,030)	(51,922)	(52,102)
Accumulated Other Comprehensive Income (Loss)
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance at beginning of period		31,100	61,338	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		(12,298)	18,639	(45,921)	23,848
Gain (loss) amounts reclassified from AOCI		1,790	(238)	(10,963)	541
Net current period OCI		(10,508)	18,401	(56,884)	24,389
Tax expense (benefit)		0	0	0	0
Total other comprehensive income (loss), net of taxes		(10,508)	18,401	(56,884)	24,389	25,149
Balance at end of period		$ 20,592	79,739	$ 20,592	79,739	80,499
Unrealized Holding Gains (Losses) on AFS Securities
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance at beginning of period			110,989		104,538	$ 104,538
OCI before reclassifications			19,197		25,116
Gain (loss) amounts reclassified from AOCI			(611)		(79)
Net current period OCI			18,586		25,037
Tax expense (benefit)			0		0
Total other comprehensive income (loss), net of taxes			18,586		25,037
Balance at end of period			$ 129,575		$ 129,575

[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.