Stockholders' Equity - Additional Information (Detail) (USD $)	12 Months Ended			0 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Oct. 21, 2014	Aug. 05, 2014	Mar. 31, 2015
Stockholders Equity Note [Line Items]
Preferred stock, shares authorized	10,000,000
Preferred stock, without par value
Proceeds from preferred stock issued during period	$ 388,500,000
Shares authorized for repurchase	7,100,000
Average price paid per share for common stock repurchased	$ 64.20	$ 55.90	$ 46.32
Through March 2015 | Upper Limit
Stockholders Equity Note [Line Items]
Stock Repurchase Authorized Amount						107,300,000
Series C Preferred Stock
Stockholders Equity Note [Line Items]
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
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