Employee Benefits - Narrative (Detail)	12 Months Ended
	Dec. 31, 2019 USD ($) Investment derivative_instrument Age	Dec. 31, 2018 USD ($) derivative_instrument	Dec. 31, 2017 USD ($)
Defined Benefit Plan Disclosure [Line Items]
Actuarial Loss (Gain)	$ 256,200,000	$ (131,400,000)
Number of investments held in diversified portfolios | Investment	1
Number of derivatives held by the plan | derivative_instrument	5	4
Expected long-term rate of return on assets	5.25%
Postretirement Health Care, minimum retirement age | Age	55
Postretirement Health Care, minimum service term	15 years
Estimated contribution defined contribution plans	$ 57,600,000	$ 54,400,000	$ 53,400,000
Trust for Benefit of Employees
Defined Benefit Plan Disclosure [Line Items]
Assets related to the pension plan	$ 128,800,000	129,900,000
International Equities
Defined Benefit Plan Disclosure [Line Items]
Target allocation of plan assets	5.00%
Northern Trust Private Equity Funds
Defined Benefit Plan Disclosure [Line Items]
Target allocation of plan assets	2.00%
Northern Trust Hedge Funds
Defined Benefit Plan Disclosure [Line Items]
Target allocation of plan assets	3.00%
Real Estate
Defined Benefit Plan Disclosure [Line Items]
Target allocation of plan assets	3.00%
Global Listed Infrastructure
Defined Benefit Plan Disclosure [Line Items]
Target allocation of plan assets	3.00%
Emerging Market Debt
Defined Benefit Plan Disclosure [Line Items]
Target allocation of plan assets	4.00%
Long Credit Bonds
Defined Benefit Plan Disclosure [Line Items]
Target allocation of plan assets	45.00%
Long Government Bonds
Defined Benefit Plan Disclosure [Line Items]
Target allocation of plan assets	10.00%
Custom Completion
Defined Benefit Plan Disclosure [Line Items]
Target allocation of plan assets	10.00%
Domestic Common Stock
Defined Benefit Plan Disclosure [Line Items]
Target allocation of plan assets	8.00%
Emerging Markets Equity
Defined Benefit Plan Disclosure [Line Items]
Target allocation of plan assets	3.00%
High Yield Bonds
Defined Benefit Plan Disclosure [Line Items]
Target allocation of plan assets	4.00%
Supplemental Plan
Defined Benefit Plan Disclosure [Line Items]
Assets related to the pension plan	$ 0	0
Prior service costs, amortization period (years)	10 years
Actuarial Loss (Gain)	$ 22,000,000.0	$ (9,700,000)
Weighted average discount rate used in determining the accumulated postretirement benefit obligation	3.37%	4.47%
Other Postretirement Plan
Defined Benefit Plan Disclosure [Line Items]
Actuarial Loss (Gain)	$ 200,000	$ (6,700,000)
Weighted average discount rate used in determining the accumulated postretirement benefit obligation	3.37%	4.47%
Current health care cost trend rate	8.25%
Ultimate health care rate	4.50%
Other Postretirement Plan | Pre-Age 65 Medical and Drug Benefits
Defined Benefit Plan Disclosure [Line Items]
Current health care cost trend rate	6.25%
Pension Plans, Defined Benefit
Defined Benefit Plan Disclosure [Line Items]
Assets related to the pension plan	$ 1,601,200,000	$ 1,380,100,000
Contributions made to the pension plan	3,000,000.0	21,900,000
Pension Plans, Defined Benefit | Northern Trust Private Equity Funds
Defined Benefit Plan Disclosure [Line Items]
Assets related to the pension plan	20,300,000	25,500,000
Pension Plans, Defined Benefit | Northern Trust Hedge Funds
Defined Benefit Plan Disclosure [Line Items]
Assets related to the pension plan	30,200,000	29,200,000
Pension Plans, Defined Benefit | Domestic Common Stock
Defined Benefit Plan Disclosure [Line Items]
Assets related to the pension plan	12,300,000	27,700,000
U.S. Plan | Pension Plans, Defined Benefit
Defined Benefit Plan Disclosure [Line Items]
Assets related to the pension plan	1,601,200,000	1,380,100,000	$ 1,506,400,000
Contributions made to the pension plan	$ 0	50,000,000.0
Prior service costs, amortization period (years)	11 years
Actuarial Loss (Gain)	$ 213,300,000	$ (112,400,000)
Expected long-term rate of return on assets	6.00%	6.00%	6.75%
Weighted average discount rate used in determining the accumulated postretirement benefit obligation	3.37%	4.47%
U.S. Plan | Pension Plans, Defined Benefit | Lower Limit
Defined Benefit Plan Disclosure [Line Items]
Contribution to plan assets	$ 0
U.S. Plan | Pension Plans, Defined Benefit | Upper Limit
Defined Benefit Plan Disclosure [Line Items]
Contribution to plan assets	$ 275,000,000.0