Intangible Assets - Summary of Changes in Carrying Amounts of Intangible Assets Including DAC, VOBA , and DSI (Detail) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017	Dec. 31, 2016
Finite-Lived Intangible Assets [Line Items]
Deferred Sales Inducement Cost, Net	$ 109	$ 106	$ 10	$ 86
VOBA
VOBA at beginning of period	821	118
Deferrals	0	0
Amortization	(75)	(34)
Interest	15	9
Unlocking	(4)	26
Adjustment for net unrealized investment (gains) losses	96	(119)
VOBA at end of period	853	0
Deferred Sales Inducement Cost, Unrealized Gain (Loss) on Investment	5	0
Deferred Sales Inducement Cost, Unlocking Adjustment	0	(2)
Deferred Sales Inducement Cost, Amortization Expense, Accrued Interest	1	3
Deferred Sales Inducement Cost, Amortization Expense	(5)	(12)
Deferred Sales Inducement Cost, Addition	98	31
DAC
DAC at beginning of period	243	1,023	22	1,024
Deferrals	219	205
Amortization	(6)	(98)
Interest	2	32
Unlocking	0	6
Adjustment for net unrealized investment (gains) losses	6	(146)
DAC at end of period	243	1,023	22	1,024
Total
Total VOBA and DAC at beginning of period	1,205	1,129	$ 853	$ 1,228
Deferrals	317	236
Amortization	(86)	(144)
Unlocking	(4)	30
Adjustment for net unrealized investment (gains) losses	107	(265)
Interest	$ 18	$ 44