Lease Commitments - Additional Information (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Leases [Abstract]
Net rental expense	$ 71.6	$ 73.2	$ 76.2
One of the buildings and related land utilized for Chicago operations	In the event of sale or refinancing, Northern Trust would anticipate receiving full repayment of any outstanding loans plus 00% of any proceeds in excess of the original project costs.
Percentage of proceeds in excess of original project costs the bank would anticipate to receive, in addition to full repayment of outstanding loan, in the event of sale or refinancing	42.00%