Commitments and Contingencies - Additional Information (Detail) - 12 months ended Dec. 31, 2017 £ in Millions, $ in Millions	USD ($) Company	GBP (£) Company
Commitments and Contingencies Disclosure [Line Items]
Indemnity limit | £		£ 28
Number of insurance company sold | Company	2	2
Commitment to fund limited partnership investments	$ 317
Commitment to fund U.S. commercial mortgage loan investments	16
Commitment to fund private placement investments	152
Notional amount
Commitments and Contingencies Disclosure [Line Items]
Commitment to fund bank loan investments	18
Rivermont Insurance Company I
Commitments and Contingencies Disclosure [Line Items]
One time commitment fee	2
Maximum potential amount of future obligation before repayment	$ 14
Margin rate	1.20%	1.20%
Long-term Care Insurance Class Action Lawsuit
Commitments and Contingencies Disclosure [Line Items]
Settlement payment	$ 20