Summary of Significant Accounting Policies - Additional Information (Details) (USD $)	12 Months Ended					12 Months Ended						1 Months Ended					12 Months Ended					1 Months Ended	12 Months Ended			0 Months Ended
	May 31, 2013	May 31, 2012	May 31, 2011	Apr. 30, 2012 Property	Nov. 30, 2011	May 31, 2013 Co-venturer	May 31, 2013 Real Estate	May 31, 2012 Real Estate	May 31, 2011 Real Estate	May 31, 2012 Cement	May 31, 2011 Cement	Nov. 30, 2011 Ready-Mix Concrete Property	Jul. 31, 2011 Ready-Mix Concrete Property	Jul. 31, 2011 Nonoperating Development Properties Property	Apr. 30, 2012 Aggregates	Jul. 31, 2011 Exchange of Productive Assets	May 31, 2011 Exchange of Productive Assets	May 31, 2013 Riverside Defined Benefit Plans Employee	May 31, 2013 Minimum	May 31, 2013 Maximum	May 31, 2013 Cement Facility	Apr. 30, 2012 Production Line	May 31, 2013 Restricted Stock Units (RSUs)	May 31, 2013 Employees Not Participating in Production-based Incentives [Member]	May 31, 2013 Certain executives	Mar. 22, 2013 Expanded shale and clay lightweight aggregates manufacturing business Property
Summary Of Significant Accounting Policies [Line Items]
Percentage of equity interest held in a joint venture	40.00%				40.00%
Fair value of long-term debt, including current portion	$ 723,200,000	$ 646,800,000
Long-term debt, carrying amount	659,807,000	658,163,000
Maturity period for investments to be classified cash and cash equivalents, days	90 days
Long-lived assets useful life																			10 years	25 years	40 years
Goodwill resulting from acquisition	40,575,000	1,715,000
Number of ready-mix concrete plants acquired																										42
Number of distribution facilities acquired																										2
Property, plant and equipment acquired in a non-cash exchange																										25,300,000
Goodwill resulting from the non-cash exchange, tax deductible amount																										38,900,000
Percentage of tax benefit likely to be realized upon ultimate settlement	50.00%
Real estate acquired for development	7,300,000	7,600,000
Life insurance contracts recorded net cash	1,100,000	700,000
Insurance policy distributions	99,800,000	94,400,000
Fair value of equity	14,900,000	13,000,000
Partner's percentage in the venture						60.00%
Percentage of joint venture's debt guaranteed	50.00%					100.00%
Equity income in joint venture	2,700,000	500,000
Deferred other charges	18,800,000	20,400,000
Debt issuance costs	11,100,000	13,000,000
Other credits	91,200,000	96,400,000
Approximate number of employees and retirees of subsidiary																		600
Accumulated other comprehensive loss, adjustments related to defined benefit retirement plan and postretirement health benefit plan, net of tax	18,400,000	24,500,000
Accumulated other comprehensive loss, adjustments related to defined benefit retirement plan and postretirement health benefit plan, tax	3,500,000	1,400,000
Gain on asset exchange	8,900,000	73,100,000	20,900,000				2,800,000	5,400,000	1,700,000	2,500,000	1,700,000				20,800,000	1,600,000	12,000,000					30,900,000
Number of assets acquired in noncash asset exchange													3
Number of assets disposed of in noncash asset exchange												7	7	4
Increase in aggregate property, plant and equipment																6,100,000	17,400,000
Award vesting period																							4 years
Equity interest in a joint venture	40.00%
Value of 40% equity interest	13,000,000
Gain recognized from the transaction	8,900,000
Number of production facilities operated				5
Oil and gas lease bonuses and royalties		1,300,000	3,100,000
Restructuring charges	0	3,153,000	0
Duration of financial incentives plans																								1 year	3 years
Financial based incentive compensation expense	$ 1,500,000	$ 5,000,000	$ 0