Industry Segment and Geographic Area Data (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015	Mar. 31, 2016
Quarter ended:
Total revenues	$ 998,911	$ 962,903	$ 1,922,320	$ 1,847,708
Depreciation and amortization, net of (gains) losses on disposals	115,893	68,721	219,216	124,481
Interest expense	28,215	23,973	54,859	46,073
Pretax earnings	278,991	287,095	512,122	556,103
Income tax expense	102,516	103,716	188,474	201,439
Identifiable assets	9,007,118	7,600,545	9,007,118	7,600,545	$ 8,127,363
United States [Member]
Quarter ended:
Total revenues	950,141	915,906	1,831,088	1,757,369
Depreciation and amortization, net of (gains) losses on disposals	114,794	68,983	216,794	125,722
Interest expense	28,213	23,973	54,857	45,962
Pretax earnings	270,644	276,263	495,557	535,390
Income tax expense	100,272	100,926	184,017	196,021
Identifiable assets	8,710,057	7,376,921	8,710,057	7,376,921
Canada [Member]
Quarter ended:
Total revenues	48,770	46,997	91,232	90,339
Depreciation and amortization, net of (gains) losses on disposals	1,099	(262)	2,422	(1,241)
Interest expense	2		2	111
Pretax earnings	8,347	10,832	16,565	20,713
Income tax expense	2,244	2,790	4,457	5,418
Identifiable assets	$ 297,061	$ 223,624	$ 297,061	$ 223,624