Industry Segment and Geographic Area Data (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended		6 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011	Mar. 31, 2012
Quarter ended:
Total revenues	$ 741,317	$ 703,181	$ 1,406,733	$ 1,346,146
Depreciation and amortization, net of (gains) losses on disposals	62,042	50,739	120,978	99,472
Interest expense	22,113	22,963	45,604	45,596
Pretax earnings	172,209	161,470	299,597	287,200
Income tax expense	62,789	60,459	109,608	107,966
Identifiable assets	5,136,964	4,403,137	5,136,964	4,403,137	4,654,051
United States
Quarter ended:
Total revenues	692,822	656,095	1,316,745	1,257,186
Depreciation and amortization, net of (gains) losses on disposals	59,972	48,689	116,935	95,553
Interest expense	21,961	22,802	45,300	45,272
Pretax earnings	162,693	152,021	282,077	270,378
Income tax expense	60,267	57,681	104,965	103,010
Identifiable assets	4,984,218	4,269,467	4,984,218	4,269,467
Canada
Quarter ended:
Total revenues	48,495	47,086	89,988	88,960
Depreciation and amortization, net of (gains) losses on disposals	2,070	2,050	4,043	3,919
Interest expense	152	161	304	324
Pretax earnings	9,516	9,449	17,520	16,822
Income tax expense	2,522	2,778	4,643	4,956
Identifiable assets	$ 152,746	$ 133,670	$ 152,746	$ 133,670