Nasdaq Stockholders' Equity (Narrative) (Details) $ / shares in Units, $ in Millions			1 Months Ended	3 Months Ended		9 Months Ended
	Sep. 11, 2020 USD ($)	Jul. 22, 2020 $ / shares	Oct. 31, 2020 USD ($) $ / shares	Sep. 30, 2020 USD ($) vote $ / shares shares	Sep. 30, 2019 $ / shares shares	Sep. 30, 2020 USD ($) vote $ / shares shares	Sep. 30, 2019 $ / shares shares	Dec. 31, 2019 vote $ / shares shares
Stockholders Equity [Line Items]
Common stock, shares authorized (in shares)				300,000,000		300,000,000		300,000,000
Common stock, shares issued (in shares)				170,298,054		170,298,054		171,075,011
Common stock, shares outstanding (in shares)				164,020,697		164,020,697		165,094,440
Common stock (in votes per share) | vote				1		1		1
Common stock holder voting rights, maximum percentage of the then-outstanding shares of Nasdaq common stock				5.00%		5.00%		5.00%
Common stock in treasury (in shares)				6,277,357		6,277,357		5,980,571
Authorized amount under share repurchase program | $				$ 446		$ 446
Number of shares of common stock repurchased (in shares)						1,752,474	2,053,855
Preferred stock, shares authorized (in shares)				30,000,000		30,000,000		30,000,000
Preferred stock par value (in dollars per share) | $ / shares				$ 0.01		$ 0.01		$ 0.01
Preferred stock, shares issued (in shares)				0	0	0	0
Preferred stock, shares outstanding (in shares)				0	0	0	0
Payments of dividends | $						$ 239
Cash dividends declared per common share (in dollars per share) | $ / shares		$ 0.49		$ 0.49	$ 0.47	$ 1.45	$ 1.38
Dividends declared | $	$ 81					$ 239
Subsequent Event
Stockholders Equity [Line Items]
Cash dividends declared per common share (in dollars per share) | $ / shares			$ 0.49
Dividends declared | $			$ 80
Other Repurchases of Common Stock
Stockholders Equity [Line Items]
Number of shares of common stock repurchased (in shares)						296,786	363,813