Contingent Liabilities - Additional Information (Detail)		1 Months Ended				3 Months Ended		12 Months Ended
	Jan. 14, 2014 case	Jan. 31, 2017 USD ($)	Jun. 30, 2016 USD ($) shares	Jun. 30, 2015 USD ($) shares	Jan. 31, 2015 trust	Sep. 30, 2016 USD ($) lawsuit	Jun. 30, 2016 USD ($) lawsuit shares	Dec. 31, 2016 USD ($) shares	Dec. 31, 2015 USD ($) shares	Dec. 31, 2013 USD ($) lawsuit
Loss Contingencies [Line Items]
Accruals recorded for pending litigation or threatened legal actions or regulatory matters						$ 4,300,000		$ 0
Legal settlement charge						$ 3,500,000	$ 46,500,000			$ 19,200,000
Number of lawsuits where settled charges have been recorded | lawsuit										2
Number of lawsuits remaining to be settled | lawsuit						1	1
Number of opt-out cases filed by merchants | case	30
Visa Class B
Loss Contingencies [Line Items]
Visa redemption gain			$ 123,100,000	$ 99,900,000				$ 123,100,000	$ 99,900,000
Common stock issued (in shares) | shares			1,100,000	1,000,000			1,100,000	1,100,000.0	1,000,000.0
Visa class b common stock shares held | shares								4,100,000	5,200,000
Remaining Visa shares held by Northern Trust, original cost basis								$ 0	$ 0
Northern Trust Fiduciary Services (Guernsey) Limited | Subsidiaries
Loss Contingencies [Line Items]
Number of administrated trusts served as trustee on | trust					2
Upper Limit
Loss Contingencies [Line Items]
Possible loss in future periods in excess of accrual								35,000,000
Lower Limit
Loss Contingencies [Line Items]
Possible loss in future periods in excess of accrual								$ 0
Subsequent Event
Loss Contingencies [Line Items]
Legal settlement charge		$ 4,300,000