Industry Segment and Geographic Area Data (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016	Mar. 31, 2017
Quarter ended:
Total revenues	$ 1,042,686	$ 998,685	$ 2,000,611	$ 1,921,876
Depreciation and amortization, net of (gains) losses on disposals	138,922	115,893	271,925	219,216
Interest expense	32,023	27,989	62,368	54,415
Pretax earnings	197,358	278,991	396,044	512,122
Income tax expense	72,719	102,516	145,198	188,474
Identifiable assets	10,082,890	9,007,118	10,082,890	9,007,118	$ 9,405,840
United States [Member]
Quarter ended:
Total revenues	988,808	949,915	1,901,922	1,830,644
Depreciation and amortization, net of (gains) losses on disposals	136,597	114,794	268,096	216,794
Interest expense	31,268	27,987	60,911	54,413
Pretax earnings	192,585	270,644	384,558	495,557
Income tax expense	71,280	100,272	141,890	184,017
Identifiable assets	9,621,634	8,710,057	9,621,634	8,710,057
Canada [Member]
Quarter ended:
Total revenues	53,878	48,770	98,689	91,232
Depreciation and amortization, net of (gains) losses on disposals	2,325	1,099	3,829	2,422
Interest expense	755	2	1,457	2
Pretax earnings	4,773	8,347	11,486	16,565
Income tax expense	1,439	2,244	3,308	4,457
Identifiable assets	$ 461,256	$ 297,061	$ 461,256	$ 297,061