Accumulated Other Comprehensive Loss - Schedule of Reclassifications out of Accumulated Other Comprehensive Loss to Net Income (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2020	Mar. 31, 2019	Mar. 31, 2018
Reclassification out of Accumulated Other Comprehensive Income [Line Items]
Amortization of net actuarial loss included in net periodic benefit cost	$ 128,704	$ 146,723	$ 148,177	$ 155,830	$ 117,880	$ 140,269	$ 119,887	$ 137,367	$ 579,434	$ 515,403	$ 430,036
Tax benefit (expense)									(96,831)	(96,874)	(128,344)
Net income	$ 138,866	$ 112,026	$ 114,325	$ 117,386	$ 89,575	$ 132,037	$ 92,713	$ 104,204	482,603	418,529	301,692
Reclassification out of Accumulated Other Comprehensive Income | Amortization of net actuarial loss
Reclassification out of Accumulated Other Comprehensive Income [Line Items]
Amortization of net actuarial loss included in net periodic benefit cost									109	2,201	2,387
Tax benefit (expense)									(28)	(576)	(860)
Net income									81	1,625	1,527
Reclassification out of Accumulated Other Comprehensive Income | Derivatives designated as cash flow hedges
Reclassification out of Accumulated Other Comprehensive Income [Line Items]
Amortization of net actuarial loss included in net periodic benefit cost									2,094	(1,026)	0
Tax benefit (expense)									(546)	269	0
Net income									$ 1,548	$ (757)	$ 0