Accumulated Other Comprehensive Income by Component (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance at beginning of period	$ 1,106,392	$ 1,291,501
Total other comprehensive income (loss), net of taxes	25,149	(1,241)	$ (52,651)
Balance at end of period	1,043,441	1,106,392	1,291,501
Unrealized Holding Gains (Losses) on AFS Securities and Cash Equivalents
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance at beginning of period	104,538
OCI before reclassifications	30,434
Amounts reclassified from AOCI	(1,440)
Net current period OCI	28,994
Tax expense (benefit)	0
Total other comprehensive income (loss), net of taxes	28,994
Balance at end of period	133,532	104,538
Unrealized Holding Gains (Losses) on Cash Flow Hedges
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance at beginning of period	(1,225)	4,202	2,296
OCI before reclassifications	(806)	(6,288)	1,807
Amounts reclassified from AOCI	1,100	861	99
Net current period OCI	294	(5,427)	1,906
Tax expense (benefit)	0	0	0
Total other comprehensive income (loss), net of taxes	294	(5,427)	1,906
Balance at end of period	(931)	(1,225)	4,202
OCI Adjustments for Policy holder liabilities, VOBA, and Deferred Tax
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance at beginning of period	(47,963)	(43,144)	(57,275)
OCI before reclassifications	(4,139)	(4,819)	14,131
Net current period OCI	(4,139)	(4,819)	14,131
Tax expense (benefit)	0	0	0
Total other comprehensive income (loss), net of taxes	(4,139)	(4,819)	14,131
Balance at end of period	(52,102)	(47,963)	(43,144)
Accumulated Other Comprehensive Income
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance at beginning of period	55,350	56,591	109,242
OCI before reclassifications	25,489	(2,226)	(52,375)
Amounts reclassified from AOCI	(340)	985	(276)
Net current period OCI	25,149	(1,241)	(52,651)
Tax expense (benefit)	0	0	0
Total other comprehensive income (loss), net of taxes	25,149	(1,241)	(52,651)
Balance at end of period	80,499	55,350	56,591
Unrealized Holding Gains (Losses) on AFS Securities
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance at beginning of period	$ 104,538	95,533	164,221
OCI before reclassifications		8,881	(68,313)
Amounts reclassified from AOCI		124	(375)
Net current period OCI		9,005	(68,688)
Tax expense (benefit)		0	0
Total other comprehensive income (loss), net of taxes		9,005	(68,688)
Balance at end of period		$ 104,538	$ 95,533