Stock plans (Details 2) (USD $)	6 Months Ended
Jun. 30, 2011	Jun. 30, 2010
Restricted share awards [Member]
Activities
Beginning balance	3,114,039	2,525,958
Granted	590,367	439,114
Vested	(553,615)	(323,520)
Forfeited	(13,198)	(22,609)
Ending balance	3,137,593	2,618,943
Weighted average grant date fair value, beginning balance	$ 24.33	$ 23.43
Weighted average grant date fair value, granted	$ 32.16	$ 26.17
Weighted average grant date fair value, vested	$ 25.46	$ 23.91
Weighted average grant date fair value, forfeited	$ 27.28	$ 23.07
Weighted average grant date fair value, ending balance	$ 25.60	$ 23.83
Restricted share units [Member]
Activities
Beginning balance	47,049	78,591
Granted	18,388	7,952
Vested	(13,340)	(59,019)
Reinvested	296
Forfeited		(1,094)
Ending balance	52,393	26,430
Weighted average grant date fair value, beginning balance	$ 25.04	$ 24.84
Weighted average grant date fair value, granted	$ 32.10	$ 26.07
Weighted average grant date fair value, vested	$ 24.72	$ 24.76
Weighted average grant date fair value, reinvested	$ 25.45
Weighted average grant date fair value, forfeited		$ 21.49
Weighted average grant date fair value, ending balance	$ 27.60	$ 25.51
Employee seller shares [Member]
Activities
Beginning balance	197,879	410,667
Granted	0	0
Vested	0	0
Forfeited	(705)	(3,551)
Ending balance	197,174	407,116
Weighted average grant date fair value, beginning balance	$ 22.01	$ 22.01
Weighted average grant date fair value, granted	$ 0.00	$ 0.00
Weighted average grant date fair value, vested	$ 0.00	$ 0.00
Weighted average grant date fair value, forfeited	$ 22.01	$ 22.01
Weighted average grant date fair value, ending balance	$ 22.01	$ 22.01
Performance share awards [Member]
Activities
Beginning balance	132,401	0
Granted	146,618	0
Vested	0	0
Forfeited	0	0
Ending balance	279,019	0
Weighted average grant date fair value, beginning balance	$ 28.70	$ 0.00
Weighted average grant date fair value, granted	$ 32.64	$ 0.00
Weighted average grant date fair value, vested	$ 0.00	$ 0.00
Weighted average grant date fair value, forfeited	$ 0.00	$ 0.00
Weighted average grant date fair value, ending balance	$ 30.77	$ 0.00

The number of equity-based payment instruments, excluding stock (or unit) options, that were forfeited during the reporting period.

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- Definition

The weighted average fair value as of the grant date of equity-based award plans other than stock (unit) option plans that were not exercised or put into effect during the reporting period as a result of the occurrence of a terminating event specified in the contractual agreement of the plan.

+ References

+ Details

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- Definition

The number of grants made during the period on other than stock (or unit) option plans (for example, phantom stock or unit plan, stock or unit appreciation rights plan, performance target plan).

+ References

+ Details

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- Definition

The weighted average fair value at grant date for nonvested equity-based awards issued during the period on other than stock (or unit) option plans (for example, phantom stock or unit plan, stock or unit appreciation rights plan, performance target plan).

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Statement of Financial Accounting Standard (FAS)
-Number 123R
-Paragraph A240
-Subparagraph b(2)(c)
-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Statement of Financial Accounting Standard (FAS)
-Number 123R
-Paragraph A240
-Subparagraph c(1)
-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)(2)(iii)(1)
-URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901

+ Details

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- Definition

The number of non-vested equity-based payment instruments, excluding stock (or unit) options, that validly exist and are outstanding as of the balance sheet date.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Statement of Financial Accounting Standard (FAS)
-Number 123R
-Paragraph A240
-Subparagraph b(2)(a)
-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Statement of Financial Accounting Standard (FAS)
-Number 123R
-Paragraph A240
-Subparagraph b(2)(b)
-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)(2)(i)-(ii)
-URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901

+ Details

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- Definition

A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period.

+ Details

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- Definition

The weighted average fair value of nonvested awards on equity-based plans excluding option plans (for example, phantom stock or unit plan, stock or unit appreciation rights plan, revenue or profit achievement stock award plan) for which the employer is contingently obligated to issue equity instruments or transfer assets to an employee who has not yet satisfied service or performance criteria necessary to gain title to proceeds from the sale of the award or underlying shares or units.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Statement of Financial Accounting Standard (FAS)
-Number 123R
-Paragraph A240
-Subparagraph b(2)(a)
-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Statement of Financial Accounting Standard (FAS)
-Number 123R
-Paragraph A240
-Subparagraph b(2)(b)
-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)(2)(i)-(ii)
-URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901

+ Details

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- Definition

The number of equity-based payment instruments, excluding stock (or unit) options, that vested during the reporting period.

+ References

+ Details

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- Definition

The weighted average fair value as of grant date pertaining to an equity-based award plan other than a stock (or unit) option plan for which the grantee gained the right during the reporting period, by satisfying service and performance requirements, to receive or retain shares or units, other instruments, or cash in accordance with the terms of the arrangement.

+ References

+ Details

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- Definition

The number of equity-based payment instruments, excluding stock (or unit) options, that were reinvested during the reporting period.

+ Details

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- Definition

The weighted average grant date fair value of restricted share units which were reinvested during the period.

+ Details

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