|
Employee Benefits (Details) (USD $)
In Millions, unless otherwise specified
|
3 Months Ended |
12 Months Ended |
|
Mar. 31, 2015
|
Mar. 31, 2014
|
Mar. 31, 2015
|
Mar. 31, 2014
|
| Qualified and nonqualified pension plans [Member] |
|
|
|
|
| Defined Benefit Plan Disclosure [Line Items] |
|
|
|
|
| Service cost |
$ 3.4 |
$ 3.0 |
$ 12.2 |
$ 13.4 |
| Interest cost |
8.1 |
8.4 |
33.1 |
31.2 |
| Expected return on plan assets |
(11.6) |
(10.6) |
(44.6) |
(39.2) |
| Prior service cost |
0 |
0.1 |
0.5 |
0.9 |
| Recognized net actuarial loss |
5.7 |
3.9 |
17.2 |
26.0 |
| Net benefit cost |
5.6 |
4.8 |
18.4 |
32.3 |
| Estimated qualified pension plan cost for current fiscal year |
18.9 |
|
18.9 |
|
| Estimated nonqualified pension plan cost for current fiscal year |
3.4 |
|
3.4 |
|
| Postretirement benefits plan [Member] |
|
|
|
|
| Defined Benefit Plan Disclosure [Line Items] |
|
|
|
|
| Service cost |
0.2 |
0.2 |
0.6 |
0.7 |
| Interest cost |
0.9 |
1.0 |
3.7 |
3.7 |
| Expected return on plan assets |
(0.8) |
(0.8) |
(3.0) |
(2.7) |
| Recognized net actuarial loss (gain) |
0.3 |
0.3 |
0.7 |
2.6 |
| Accretion of regulatory liability |
0.2 |
0.2 |
0.8 |
0.5 |
| Net benefit cost |
0.8 |
0.9 |
2.8 |
4.8 |
| Estimated postretirement benefits cost for current fiscal year, before accretion of regulatory liability |
$ 2.2 |
|
$ 2.2 |
|