Accumulated Other Comprehensive Income by Component (Detail) - USD ($) $ in Thousands		3 Months Ended		12 Months Ended
		Mar. 31, 2018	Mar. 31, 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		(46,376)	$ 5,988
Balance at end of period		824,077		$ 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		133,532
Cumulative effect adjustment	[1]	(3,023)
Adjusted balance at beginning of period		130,509
OCI before reclassifications		(44,199)
Gain (loss) amounts reclassified from AOCI		(11,450)
Net current period OCI		(55,649)
Tax expense (benefit)		0
Total other comprehensive income (loss), net of taxes		(55,649)
Balance at end of period		74,860		133,532
Unrealized Holding Gains (Losses) on Cash Flow Hedges
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance at beginning of period		(931)	(1,225)	(1,225)
Adjusted balance at beginning of period		(931)
OCI before reclassifications		1,792	161
Gain (loss) amounts reclassified from AOCI		(1,303)	247
Net current period OCI		489	408
Tax expense (benefit)		0	0
Total other comprehensive income (loss), net of taxes		489	408
Balance at end of period		(442)	(817)	(931)
OCI Adjustments for Policy holder liabilities, VOBA, and Deferred Tax
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance at beginning of period		(52,102)	(47,963)	(47,963)
Adjusted balance at beginning of period		(52,102)
OCI before reclassifications		8,784	(871)
Net current period OCI		8,784	(871)
Tax expense (benefit)		0	0
Total other comprehensive income (loss), net of taxes		8,784	(871)
Balance at end of period		(43,318)	(48,834)	(52,102)
Accumulated Other Comprehensive Income (Loss)
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance at beginning of period		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		(33,623)	5,209
Gain (loss) amounts reclassified from AOCI		(12,753)	779
Net current period OCI		(46,376)	5,988
Tax expense (benefit)		0	0
Total other comprehensive income (loss), net of taxes		(46,376)	5,988	25,149
Balance at end of period		$ 31,100	61,338	80,499
Unrealized Holding Gains (Losses) on AFS Securities
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance at beginning of period			104,538	$ 104,538
OCI before reclassifications			5,919
Gain (loss) amounts reclassified from AOCI			532
Net current period OCI			6,451
Tax expense (benefit)			0
Total other comprehensive income (loss), net of taxes			6,451
Balance at end of period			$ 110,989

[1] See Note 1 Summary of Significant Accounting Policies, for discussion on the adjustment relating to ASU 2016-01

[2] See Note 1 Summary of Significant Accounting Policies, for discussion on the adjustment relating to Accounting Standards Update 2016-01.