Employee Benefits - Narrative (Detail)	12 Months Ended
	Dec. 31, 2018 USD ($) Investment Age derivative_instruments	Dec. 31, 2017 USD ($) derivative_instruments	Dec. 31, 2016 USD ($)
Defined Benefit Plan Disclosure [Line Items]
Actuarial gain (loss)	$ 131,400,000	$ (164,500,000)
Number of investments held in diversified portfolios | Investment	2
Number of derivatives held by the plan | derivative_instruments	4	4
Expected long-term rate of return on assets	6.00%
Postretirement Health Care, minimum retirement age | Age	55
Postretirement Health Care, minimum service term	15 years
Estimated contribution defined contribution plans	$ 54,400,000	$ 53,400,000	$ 50,000,000
Trust for Benefit of Employees
Defined Benefit Plan Disclosure [Line Items]
Assets related to the pension plan	129,900,000	116,700,000
Contributions made to the pension plan	$ 21,900,000	11,500,000
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)	9 years
Actuarial gain (loss)	$ 9,700,000	$ (21,500,000)
Weighted average discount rate used in determining the accumulated postretirement benefit obligation	4.47%	3.79%
Other Postretirement Plan
Defined Benefit Plan Disclosure [Line Items]
Actuarial gain (loss)	$ 6,700,000	$ 200,000
Weighted average discount rate used in determining the accumulated postretirement benefit obligation	4.47%	3.79%
Current health care cost trend rate	8.50%
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	6.50%
Pension Plans, Defined Benefit
Defined Benefit Plan Disclosure [Line Items]
Assets related to the pension plan	$ 1,380,100,000	$ 1,506,400,000
Pension Plans, Defined Benefit | Northern Trust Private Equity Funds
Defined Benefit Plan Disclosure [Line Items]
Assets related to the pension plan	25,500,000	29,300,000
Pension Plans, Defined Benefit | Northern Trust Hedge Funds
Defined Benefit Plan Disclosure [Line Items]
Assets related to the pension plan	29,200,000	44,600,000
Pension Plans, Defined Benefit | Domestic Common Stock
Defined Benefit Plan Disclosure [Line Items]
Assets related to the pension plan	36,300,000	38,600,000
U.S. Plan | Pension Plans, Defined Benefit
Defined Benefit Plan Disclosure [Line Items]
Assets related to the pension plan	1,380,100,000	1,506,400,000	$ 1,393,500,000
Contributions made to the pension plan	$ 50,000,000	0
Prior service costs, amortization period (years)	11 years
Actuarial gain (loss)	$ 112,400,000	$ (142,600,000)
Expected long-term rate of return on assets	6.00%	6.75%	7.00%
Weighted average discount rate used in determining the accumulated postretirement benefit obligation	4.47%	3.79%
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	$ 270,000,000