Stockholders' Equity - Additional Information (Detail) - USD ($)		12 Months Ended
	Aug. 05, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013	Jun. 30, 2016
Class of Stock [Line Items]
Preferred stock, shares authorized		10,000,000	10,000,000
Proceeds from preferred stock issued during period		$ 0	$ 388,500,000	$ 0
Shares authorized for repurchase		9,700,000
Average price paid per share for common stock repurchased (in dollars per share)		$ 72.52	$ 64.20	$ 55.90
Through March 2015 | Upper Limit
Class of Stock [Line Items]
Stock repurchase authorized amount					$ 285,300,000
Series C Preferred Stock [Member]
Class of Stock [Line Items]
Stock issued during period	16,000,000
Per share ownership interest, percentage	0.10%
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, 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