Long-Term Obligations - Additional Information (Detail) (USD $)	1 Months Ended	6 Months Ended	12 Months Ended
	Oct. 31, 2007	Jun. 30, 2012 Year	Dec. 31, 2011
Debt Instrument [Line Items]
Revolving credit agreement, maximum borrowing capacity		$ 450,000,000
Single draw term loan facility, maximum borrowing capacity		250,000,000
Line of credit facility, initiation date		Jun. 29, 2012
Revolving credit facility, years		5
Single draw term loan facility, years		5
Letter of credit sub-facility		60,000,000
Borrowings outstanding under credit facility		0
Standby letters of credit issued		36,800,000
Single draw term loan facility, outstanding		250,000,000
Percentage of principal amortizing five year period		50.00%
Debt instrument, frequency of fee		quarterly
Letter of credit facility fee, minimum range, percentage		0.175%
Letter of credit facility fee, maximum range, percentage		0.275%
Interest rate per annum added to default rate		2.00%
Base conversion rate		31.0828
Base conversion price, per share		$ 32.17
Convertible debt, interest rate		2.75%
2.75% Series A [Member]
Debt Instrument [Line Items]
Convertible senior debentures, issuance date	Oct. 31, 2007
Convertible debt, principal amount	275,000,000	275,000,000	275,000,000
Exercise of option over-allotment amount	25,000,000
Convertible debt, maturity date		2037
Debentures principal amount for conversion calculation		1,000
Convertible debt, right to redeem, earliest date		Nov. 01, 2012
Number of years within which holders to put securities for cash		1	1
Debt instrument, rate of return without conversion feature		7.00%
Debt instrument expected lives, years		5
Convertible debt, proceeds excess fair value of liability reclassified to equity	47,400,000
Interest expense accretion period, in years		5
Remaining period that discount will be amortized, months		4 months
2.75% Series B [Member]
Debt Instrument [Line Items]
Convertible senior debentures, issuance date	Oct. 31, 2007
Convertible debt, principal amount	275,000,000	275,000,000	275,000,000
Exercise of option over-allotment amount	25,000,000
Convertible debt, maturity date		2037
Debentures principal amount for conversion calculation		1,000
Convertible debt, right to redeem, earliest date		Nov. 01, 2014
Debt instrument, rate of return without conversion feature		7.40%
Debt instrument expected lives, years		7
Convertible debt, proceeds excess fair value of liability reclassified to equity	67,200,000
Interest expense accretion period, in years		7
Remaining period that discount will be amortized, months		28 months
Series Debentures [Member]
Debt Instrument [Line Items]
Debentures principal amount for conversion calculation		$ 1,000