Partnership Capital Subject to Mandatory Redemption - Roll Forward of Outstanding Partnership Loans (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Other Liabilities Disclosure [Abstract]
Partnership loans outstanding at beginning of year	$ 297	$ 266
Partnership loans issued during the year	170	142
Repayment of Partnership loans during the year	(135)	(111)
Partnership loans outstanding at end of year	$ 332	$ 297