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	$ 1,240.7	$ 798.1
Prior service cost (credit)	10.5	4.7
Total	1,251.2	802.8
Net actuarial loss	87.2
Prior service costs (credits)	3.0
Total 2017 - estimated amortization	90.2
Components of net periodic benefit cost (including amounts capitalized to the balance sheets)
Service cost	45.4	30.4	$ 10.1
Interest cost	130.8	94.3	68.1
Expected return on plan assets	(195.9)	(155.6)	(98.6)
Plan settlement	16.5	0.0	0.0
Plan curtailment	0.0	(0.3)	0.0
Amortization of prior service cost (credit)	3.4	2.2	2.1
Amortization of net actuarial loss	82.9	68.5	36.7
Net periodic benefit cost	83.1	39.5	18.4
OPEB
Net regulatory assets
Net actuarial loss	25.8	23.7
Prior service cost (credit)	(87.9)	(3.3)
Total	(62.1)	20.4
Net actuarial loss	5.8
Prior service costs (credits)	(11.2)
Total 2017 - estimated amortization	(5.4)
Components of net periodic benefit cost (including amounts capitalized to the balance sheets)
Service cost	26.1	20.7	8.5
Interest cost	37.0	26.7	17.8
Expected return on plan assets	(52.7)	(39.6)	(23.7)
Plan settlement	0.0	0.0	0.0
Plan curtailment	0.0	0.0	0.0
Amortization of prior service cost (credit)	(9.4)	(6.4)	(1.8)
Amortization of net actuarial loss	8.5	3.9	1.2
Net periodic benefit cost	9.5	5.3	$ 2.0
Nonutility operations | Pension Benefits
Employee Benefit Plans
Net actuarial loss (gain)	12.0	11.4
Total	12.0	11.4
Nonutility operations | OPEB
Employee Benefit Plans
Net actuarial loss (gain)	(1.0)	(0.6)
Total	$ (1.0)	$ (0.6)