Interest Income and Interest Expense (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Interest income:
Cash and equivalents	$ 2	$ 1	$ 4	$ 3
Cash required to be segregated under federal or other regulations	3	1	6	2
Available-for-sale securities	95	68	180	132
Held-to-maturity securities	137	107	257	210
Margin receivables	75	61	141	125
Loans	41	49	84	100
Broker-related receivables and other	1	1	1	1
Subtotal interest income	354	288	673	573
Other interest revenue(1)	24	18	46	41
Total interest income	378	306	719	614
Interest expense:
Deposits	(1)	(1)	(2)	(2)
Customer payables	(2)	(1)	(3)	(2)
Other borrowings	(5)	(4)	(10)	(9)
Corporate debt	(13)	(14)	(27)	(27)
Subtotal interest expense	(21)	(20)	(42)	(40)
Other interest expense(2)	(1)	0	(2)	(1)
Total interest expense	(22)	(20)	(44)	(41)
Net interest income	$ 356	$ 286	$ 675	$ 573