Consolidated Statements of Cash Flows - USD ($) $ in Millions	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017
Operating activities:
Net income	$ 462	$ 427	$ 936	$ 938	$ 2,554
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization	143	142	281	276
Stock-based compensation			61	68
Deferred taxes			(7)	(11)
Cetip realized investment gain, net			0	(114)
Other			(4)	(7)
Changes in assets and liabilities:
Customer accounts receivable			(148)	(170)
Other current and non-current assets			(39)	(37)
Section 31 fees payable			80	51
Deferred revenue			249	240
Other current and non-current liabilities			(173)	(135)
Total adjustments			300	161
Net cash provided by operating activities			1,236	1,099
Investing activities:
Capital expenditures			(33)	(81)
Capitalized software development costs			(75)	(69)
Proceeds from sale of Cetip, net			0	438
Cash paid for acquisitions, net of cash received for divestiture			(405)	10
Purchases of investments			(305)	0
Net cash provided by (used in) investing activities			(818)	298
Financing activities:
Proceeds from (repayments of) commercial paper, net			812	(469)
Repurchases of common stock			(759)	(469)
Dividends to stockholders			(279)	(239)
Payments relating to treasury shares received for restricted stock tax payments and stock option exercises			(76)	(81)
Acquisition of non-controlling interest and redeemable non-controlling interest			0	(55)
Other			1	(8)
Net cash used in financing activities			(301)	(1,321)
Effect of exchange rate changes on cash, cash equivalents and restricted cash and cash equivalents			(5)	5
Net increase in cash, cash equivalents, and restricted cash and cash equivalents			112	81
Cash, cash equivalents, and restricted cash and cash equivalents, beginning of period			1,568	1,350	1,350
Cash, cash equivalents, and restricted cash and cash equivalents, end of period	$ 1,680	$ 1,431	1,680	1,431	$ 1,568
Supplemental cash flow disclosure:
Cash paid for income taxes			346	429
Cash paid for interest			$ 104	$ 87