Partnership Capital - Roll Forward of Outstanding Partnership Loans (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Partners Capital Notes [Abstract]
Partnership loans outstanding at beginning of year	$ 266	$ 218
Partnership loans issued during the year	142	146
Repayment of Partnership loans during the year	(111)	(98)
Partnership loans outstanding at end of year	$ 297	$ 266