Employee Benefits - Net Periodic Benefit Cost (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Pension Costs
Components of net periodic benefit cost (including amounts capitalized to the balance sheets)
Service cost	$ 47.1	$ 44.6	$ 45.4
Interest cost	114.3	121.8	130.8
Expected return on plan assets	(196.5)	(195.7)	(195.9)
Plan settlement	1.0	9.0	16.5
Amortization of prior service cost (credit)	2.7	2.9	3.4
Amortization of net actuarial loss	94.0	86.1	82.9
Net periodic benefit cost (credit)	62.6	68.7	83.1
OPEB Costs
Components of net periodic benefit cost (including amounts capitalized to the balance sheets)
Service cost	23.7	24.1	26.1
Interest cost	29.9	32.9	37.0
Expected return on plan assets	(59.5)	(55.5)	(52.7)
Plan settlement	0.0	0.0	0.0
Amortization of prior service cost (credit)	(15.4)	(12.3)	(9.4)
Amortization of net actuarial loss	1.0	3.1	8.5
Net periodic benefit cost (credit)	$ (20.3)	$ (7.7)	$ 9.5