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Pension and Other Benefit Plans - Pension Plan, Net Periodic Costs and Other Changes (Details) - Defined Benefit Pension Plans - USD ($)
$ in Millions
3 Months Ended 6 Months Ended
Sep. 30, 2017
Sep. 30, 2016
Sep. 30, 2017
Sep. 30, 2016
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]        
Service cost $ 34 $ 5 $ 66 $ 11
Interest cost 61 21 121 42
Expected return on assets (132) (40) (260) (83)
Amortization of prior service costs (4) (4) (8) (9)
Contractual termination benefit 2 0 11 0
Net periodic pension expense (income) $ (39) $ (18) $ (70) $ (39)
Defined Benefit Plan, Assumptions Used in Calculations [Abstract]        
Discount or settlement rates     2.50% 3.10%
Expected long-term rates of return on assets     4.90% 6.30%
Rates of increase in compensation levels     2.70% 2.60%