Contractual interest rate for funds borrowed, under the debt agreement.

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Borrowings - Additional Information (Detail) - USD ($) $ in Millions		1 Months Ended	6 Months Ended
	Jul. 31, 2018	Jan. 31, 2018	Jun. 30, 2018	Dec. 31, 2017
Debt Instrument [Line Items]
Junior subordinated debentures			$ 521
Debt Maturity, Start Year			Jan. 01, 2027
Debt Maturity, End Year			Dec. 31, 2033
Agreements to repurchase securities			$ 416	$ 422
Collateral posted			$ 433	$ 442
Bank Note Program [Member]
Debt Instrument [Line Items]
Senior notes		$ 1,000
Debt instrument maturity year		Jan. 31, 2021
Bank Note Program Fixed Interest Rate [Member]
Debt Instrument [Line Items]
Senior notes		$ 650
Fixed interest rate		2.625%
Subsequent Event [Member]
Debt Instrument [Line Items]
Senior notes	$ 750
Debt instrument maturity year	Jul. 31, 2023
Subsequent Event [Member] \| Fixed Interest Rate [Member]
Debt Instrument [Line Items]
Senior notes	$ 500
Fixed interest rate	3.55%
London Interbank Offered Rate (LIBOR) [Member] \| Bank Note Program Variable Interest Rate [Member]
Debt Instrument [Line Items]
Senior notes		$ 350
London Interbank Offered Rate (LIBOR) [Member] \| Subsequent Event [Member] \| Variable Interest Rate [Member]
Debt Instrument [Line Items]
Senior notes	$ 250