Summary of Significant Accounting Policies (Details) (USD $)	12 Months Ended
Dec. 31, 2010	Dec. 31, 2009	Dec. 31, 2008
Weighted average assumptions used to estimate the fair value for the Company's share options
Expected volatility	32.40%	26.80%	20.30%
Expected life	5	5	5
Expected dividend yield	4.85%	4.68%	4.95%
Risk-free interest rate	2.29%	1.89%	2.67%
Option valuation per share	$ 6.18	$ 3.38	$ 4.08
Short-term Debt [Line Items]
Unsecured Notes, face amount	650,000,000
Convertible Notes, stated interest rate		5.60%
Convertible Notes Payable [Member]
Short-term Debt [Line Items]
Unsecured Notes, face amount	650,000,000
Convertible Notes, issuance date	Aug-06
Convertible Notes, maturity date	Aug. 15, 2026
Effective interest rate for conversion option	5.80%
Interest expense recognized in relation to nonconvertible debt	$ 18,600,000	$ 20,600,000	$ 24,400,000
Convertible Notes, stated interest rate	3.85%	3.85%	3.85%

The effective interest rate on the liability component of convertible debt instrument which may be settled in cash upon conversion, including partial cash settlement.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
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Interest expense related to convertible debt instruments which has been recognized for the period, including the contractual interest coupon and amortization of the debt discount, if any.

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Reference 1: http://www.xbrl.org/2003/role/presentationRef
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The stated principal amount of the debt instrument at time of issuance, which may vary from the carrying amount because of unamortized premium or discount.

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Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher AICPA
-Name Accounting Principles Board Opinion (APB)
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Interest rate stated in the contractual debt agreement.

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Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher SEC
-Name Regulation S-X (SX)
-Number 210
-Section 02
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Date when the debt instrument was issued, which may be presented in a variety of ways (year, month and year, day, month and year, quarter, etc.).

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Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher SEC
-Name Regulation S-X (SX)
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Reference 1: http://www.xbrl.org/2003/role/presentationRef
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Reference 2: http://www.xbrl.org/2003/role/presentationRef
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The estimated dividend rate (a percentage of the share price) to be paid (expected dividends) to holders of the underlying shares over the option's term.

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Reference 1: http://www.xbrl.org/2003/role/presentationRef
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The period of time a share-based award is expected to be outstanding. A share-based award's expected term shall be determined based on, among other factors, the instrument's contractual term and the effects of employees' expected exercise and post-vesting employment termination behavior. An entity is required to aggregate individual awards into relatively homogeneous groups.

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Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
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-Paragraph A240
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Reference 2: http://www.xbrl.org/2003/role/presentationRef
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The estimated measure of the percentage amount by which a share price is expected to fluctuate during a period. Volatility also may be defined as a probability-weighted measure of the dispersion of returns about the mean. The volatility of a share price is the standard deviation of the continuously compounded rates of return on the share over a specified period. That is the same as the standard deviation of the differences in the natural logarithms of the stock prices plus dividends, if any, over the period.

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The risk-free interest rate assumption that is used in valuing an option on its own shares.

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Reference 1: http://www.xbrl.org/2003/role/presentationRef
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Disclosure of the weighted average expected dividend for an entity using a valuation technique with different dividend rates during the contractual term.

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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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