Benefit Plans - Additional Information (Detail) - USD ($) $ in Millions |
3 Months Ended | 9 Months Ended | |
|---|---|---|---|
Jun. 30, 2015 |
Jun. 30, 2014 |
Sep. 30, 2015 |
|
| Defined Benefit Plan Disclosure [Line Items] | |||
| Eligible age for several postretirement benefit plans | 55 years | ||
| Eligible age for Medicare benefits | 65 years | ||
| Defined benefit plan obligation, curtailment | $ 55 | ||
| Gain recognized due to curtailment | $ 86 | ||
| Maximum [Member] | |||
| Defined Benefit Plan Disclosure [Line Items] | |||
| Defined benefit plan obligation, discount rate | 4.00% | 3.60% | |
| Minimum [Member] | |||
| Defined Benefit Plan Disclosure [Line Items] | |||
| Defined benefit plan obligation, discount rate | 3.90% | 3.10% | |