Issuance of Preferred Stock - Additional Information (Detail) (USD $)	9 Months Ended	0 Months Ended	9 Months Ended		0 Months Ended
	Sep. 30, 2014	Aug. 05, 2014 Series C Preferred Stock	Sep. 30, 2014 Series C Preferred Stock	Aug. 05, 2014 Series C Preferred Stock	Oct. 21, 2014 Series C Preferred Stock Subsequent Event
Class of Stock [Line Items]
Preferred stock, shares authorized	10,000,000
Stock issued during period		16,000,000
Per share ownership interest, percentage				0.10%
Preferred stock, without par value
Preferred stock, liquidation preference				$ 25,000
Depository shares, liquidation preference per share				$ 25
Proceeds from preferred stock issued during period	$ 388,500,000	$ 388,500,000
Preferred stock, annual dividend rate			5.85%
Preferred Stock, Dividends Per Share, Declared					$ 593.125
Dividends Payable, Date to be Paid					Jan. 01, 2015
Dividends Payable, Date of Record					Dec. 15, 2014
Preferred Stock, redemption start date			Oct. 01, 2019
Preferred Stock, redemption terms			The Series C Preferred Stock is redeemable at Northern Trust's option, in whole or in part, on any dividend payment date on or after October 1, 2019. The Series C Preferred stock is redeemable at the Company's option, in whole, but not in part, prior to October 1, 2019 within 90 days of a regulatory capital treatment event
Preferred stock redemption period following regulatory capital treatment event			90 days