Employee Benefit Plans (Details 6) - USD ($) $ in Thousands	9 Months Ended	12 Months Ended
	Dec. 31, 2015	Mar. 31, 2015	Mar. 31, 2014
Contributions
Employer contributions	$ 41,028	$ 51,205	$ 48,379
Pension Benefits
Contributions
Employer contributions	73,844	$ 87,150
Expected employer's contribution in next fiscal year	37,000
Expected Future Benefit Payments
2016	189,200
2017	191,900
2018	195,800
2019	200,800
2020	204,400
2021 through 2025	$ 1,043,300
Other Postretirement Benefits
Defined Benefit Plans
Percentage of postretirement benefit obligations that were pre-funded as of period end	52.50%	48.40%
Percentage of pension plan assets held in 401 (h) account	44.00%	44.00%
Contributions
Employer contributions directly to the pension trust for the period	$ 5,644
Employer contributions	5,644	$ 9,769
Expected employer's contribution in next fiscal year	7,552
Expected Future Benefit Payments
2016	10,547
2017	10,359
2018	10,106
2019	9,801
2020	9,482
2021 through 2025	41,164
Supplemental (nonqualified) executive retirement plan
Contributions
Employer contributions directly to the pension trust for the period	72,000
Employer's contribution to retirees during the period	1,845
Expected employer's contribution directly to retirees in next fiscal year	$ 5,400