Industry Segment and Geographic Area Data (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Mar. 31, 2018
Quarter ended:
Total revenues	$ 1,104,507	$ 1,042,686	$ 2,124,084	$ 2,000,611
Depreciation and amortization, net of (gains) losses on disposals	138,524	138,922	270,982	271,925
Interest expense	35,030	32,023	70,284	62,368
Pretax earnings (loss)	215,660	197,358	384,857	396,044
Income tax expense	52,118	72,719	93,466	145,198
Identifiable assets	11,323,845	10,082,890	11,323,845	10,082,890	$ 10,747,422
United States [Member]
Quarter ended:
Total revenues	1,049,846	988,808	2,021,141	1,901,922
Depreciation and amortization, net of (gains) losses on disposals	137,527	136,597	268,906	268,096
Interest expense	34,324	31,268	68,873	60,911
Pretax earnings (loss)	208,208	192,625	371,470	384,558
Income tax expense	50,107	71,280	89,884	141,890
Identifiable assets	10,963,411	9,621,634	10,963,411	9,621,634
Canada [Member]
Quarter ended:
Total revenues	54,661	53,878	102,943	98,689
Depreciation and amortization, net of (gains) losses on disposals	997	2,325	2,076	3,829
Interest expense	706	755	1,411	1,457
Pretax earnings (loss)	7,452	4,733	13,387	11,486
Income tax expense	2,011	1,439	3,582	3,308
Identifiable assets	$ 360,434	$ 461,256	$ 360,434	$ 461,256