Accumulated Other Comprehensive Income by Component (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016	Dec. 31, 2016
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance at beginning of period			$ 1,106,392
Total other comprehensive income (loss), net of taxes	$ (521)	$ (3,825)	23,868	$ 71,258
Balance at end of period	1,169,387		1,169,387		$ 1,106,392
Unrealized Holding Gains (Losses) on AFS Securities
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance at beginning of period	129,575	182,576	104,538	95,533	95,533
OCI before reclassifications	1,999	268	27,115	88,922
Amounts reclassified from AOCI		1,775	(79)	164
Net current period OCI	1,999	2,043	27,036	89,086
Tax expense (benefit)	0	0	0	0
Total other comprehensive income (loss), net of taxes	1,999	2,043	27,036	89,086
Balance at end of period	131,574	184,619	131,574	184,619	104,538
Unrealized Holding Gains (Losses) on Cash Flow Hedges
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance at beginning of period	2,086	4,658	(1,225)	4,202	4,202
OCI before reclassifications	(1,803)	(3,101)	888	(2,961)
Amounts reclassified from AOCI	63	305	683	621
Net current period OCI	(1,740)	(2,796)	1,571	(2,340)
Tax expense (benefit)	0	0	0	0
Total other comprehensive income (loss), net of taxes	(1,740)	(2,796)	1,571	(2,340)
Balance at end of period	346	1,862	346	1,862	(1,225)
OCI Adjustments for Policy holder liabilities, VOBA, and Deferred Tax
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance at beginning of period	(51,922)	(55,560)	(47,963)	(43,144)	(43,144)
OCI before reclassifications	(780)	(3,072)	(4,739)	(15,488)
Net current period OCI	(780)	(3,072)	(4,739)	(15,488)
Tax expense (benefit)	0	0	0	0
Total other comprehensive income (loss), net of taxes	(780)	(3,072)	(4,739)	(15,488)
Balance at end of period	(52,702)	(58,632)	(52,702)	(58,632)	(47,963)
Accumulated Other Comprehensive Income
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance at beginning of period	79,739	131,674	55,350	56,591	56,591
OCI before reclassifications	(584)	(5,905)	23,264	70,473
Amounts reclassified from AOCI	63	2,080	604	785
Net current period OCI	(521)	(3,825)	23,868	71,258
Tax expense (benefit)	0	0	0	0
Total other comprehensive income (loss), net of taxes	(521)	(3,825)	23,868	71,258	(1,241)
Balance at end of period	$ 79,218	$ 127,849	$ 79,218	$ 127,849	$ 55,350