Employee Benefits - Net Periodic Benefit Cost (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Pension Benefits
Net regulatory assets
Net actuarial loss	$ 518.5	$ 520.9
Prior service cost (credit)	0.2	4.3
Total	518.7	525.2
Net actuarial loss	35.4
Prior service costs (credit)	1.1
Total 2017 - estimated amortization	36.5
Components of net periodic benefit cost (including amounts capitalized to the balance sheets)
Service cost	10.5	14.7	$ 9.4
Interest cost	49.7	52.9	59.3
Expected return on plan assets	(77.7)	(83.6)	(79.1)
Amortization of prior service cost (credit)	1.6	2.0	2.0
Amortization of net actuarial loss	32.4	35.6	26.9
Net periodic benefit cost	16.5	21.6	18.5
OPEB
Net regulatory assets
Net actuarial loss	4.6	14.7
Prior service cost (credit)	(3.0)	(4.1)
Total	1.6	10.6
Net actuarial loss	1.0
Prior service costs (credit)	(1.1)
Total 2017 - estimated amortization	(0.1)
Components of net periodic benefit cost (including amounts capitalized to the balance sheets)
Service cost	7.3	9.0	8.1
Interest cost	13.2	13.4	14.4
Expected return on plan assets	(14.0)	(16.0)	(16.2)
Amortization of prior service cost (credit)	(1.1)	(1.1)	(1.7)
Amortization of net actuarial loss	1.0	1.0	0.2
Net periodic benefit cost	$ 6.4	$ 6.3	$ 4.8