Industry Segment and Geographic Area Data (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019	Mar. 31, 2020
Quarter ended:
Total revenues	$ 1,324,908	$ 1,150,214	$ 2,312,145	$ 2,229,463
Depreciation and amortization, net of (gains) losses on disposals	146,415	157,851	318,718	302,893
Interest expense	40,525	39,122	80,046	78,010
Pretax earnings (loss)	351,019	205,354	465,337	380,068
Income tax expense	84,654	49,028	111,246	91,320
Identifiable assets	14,123,478	12,856,202	14,123,478	12,856,202	$ 13,438,024
United States [Member]
Quarter ended:
Total revenues	1,261,235	1,092,762	2,204,038	2,121,336
Depreciation and amortization, net of (gains) losses on disposals	144,222	154,968	312,748	296,866
Interest expense	39,983	38,261	78,637	76,481
Pretax earnings (loss)	340,200	200,112	452,149	370,959
Income tax expense	81,624	47,534	107,407	88,648
Identifiable assets	13,693,016	12,441,917	13,693,016	12,441,917
Canada [Member]
Quarter ended:
Total revenues	63,673	57,452	108,107	108,127
Depreciation and amortization, net of (gains) losses on disposals	2,193	2,883	5,970	6,027
Interest expense	542	861	1,409	1,529
Pretax earnings (loss)	10,819	5,242	13,188	9,109
Income tax expense	3,030	1,494	3,839	2,672
Identifiable assets	$ 430,462	$ 414,285	$ 430,462	$ 414,285