Share-Based Compensation Plans - Summary of options outstanding and exercisable (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	12 Months Ended
Dec. 31, 2014
Share options outstanding and share options exercisable
Share options, outstanding (in shares)	2,300
Share options exercisable (in shares)	2,273
$19.54 - $22.86 [Member]
Share options outstanding and share options exercisable
Share options, range of exercise prices, low end of the range (in dollars per share)	$ 19.54
Share options, range of exercise prices, high end of the range (in dollars per share)	$ 22.86
Share options, outstanding (in shares)	1,089
Weighted average remaining contractual life of share options outstanding	5 months 27 days
Weighted average exercise price of share options outstanding (in dollars per share)	$ 22.64
Share options exercisable (in shares)	1,089
Weighted average remaining contractual life of share options exercisable (in years)	5 months 27 days
Weighted average exercise price of share options exercisable (in dollars per share)	$ 22.64
$22.87 - $25.51 [Member]
Share options outstanding and share options exercisable
Share options, range of exercise prices, low end of the range (in dollars per share)	$ 22.87
Share options, range of exercise prices, high end of the range (in dollars per share)	$ 25.51
Share options, outstanding (in shares)	71
Weighted average remaining contractual life of share options outstanding	5 months 27 days
Weighted average exercise price of share options outstanding (in dollars per share)	$ 25.35
Share options exercisable (in shares)	71
Weighted average remaining contractual life of share options exercisable (in years)	5 months 27 days
Weighted average exercise price of share options exercisable (in dollars per share)	$ 25.35
$25.52 - $32.53 [Member]
Share options outstanding and share options exercisable
Share options, range of exercise prices, low end of the range (in dollars per share)	$ 25.52
Share options, range of exercise prices, high end of the range (in dollars per share)	$ 32.53
Share options, outstanding (in shares)	34
Weighted average remaining contractual life of share options outstanding	2 years 8 months 23 days
Weighted average exercise price of share options outstanding (in dollars per share)	$ 29.15
Share options exercisable (in shares)	34
Weighted average remaining contractual life of share options exercisable (in years)	2 years 8 months 23 days
Weighted average exercise price of share options exercisable (in dollars per share)	$ 29.15
$32.54 - $36.88 [Member]
Share options outstanding and share options exercisable
Share options, range of exercise prices, low end of the range (in dollars per share)	$ 32.54
Share options, range of exercise prices, high end of the range (in dollars per share)	$ 36.88
Share options, outstanding (in shares)	205
Weighted average remaining contractual life of share options outstanding	1 year 11 months 5 days
Weighted average exercise price of share options outstanding (in dollars per share)	$ 35.82
Share options exercisable (in shares)	205
Weighted average remaining contractual life of share options exercisable (in years)	1 year 11 months 5 days
Weighted average exercise price of share options exercisable (in dollars per share)	$ 35.82
$36.89 - $43.44 [Member]
Share options outstanding and share options exercisable
Share options, range of exercise prices, low end of the range (in dollars per share)	$ 36.89
Share options, range of exercise prices, high end of the range (in dollars per share)	$ 43.44
Share options, outstanding (in shares)	481
Weighted average remaining contractual life of share options outstanding	2 years 4 months 10 days
Weighted average exercise price of share options outstanding (in dollars per share)	$ 39.29
Share options exercisable (in shares)	481
Weighted average remaining contractual life of share options exercisable (in years)	2 years 4 months 10 days
Weighted average exercise price of share options exercisable (in dollars per share)	$ 39.29
$43.45 - $47.16 [Member]
Share options outstanding and share options exercisable
Share options, range of exercise prices, low end of the range (in dollars per share)	$ 43.45
Share options, range of exercise prices, high end of the range (in dollars per share)	$ 47.16
Share options, outstanding (in shares)	265
Weighted average remaining contractual life of share options outstanding	1 year 9 months 7 days
Weighted average exercise price of share options outstanding (in dollars per share)	$ 45.81
Share options exercisable (in shares)	265
Weighted average remaining contractual life of share options exercisable (in years)	1 year 9 months 7 days
Weighted average exercise price of share options exercisable (in dollars per share)	$ 45.81
$47.47 - $52.93 [Member]
Share options outstanding and share options exercisable
Share options, range of exercise prices, low end of the range (in dollars per share)	$ 47.17
Share options, range of exercise prices, high end of the range (in dollars per share)	$ 52.93
Share options, outstanding (in shares)	155
Weighted average remaining contractual life of share options outstanding	4 years 8 months 9 days
Weighted average exercise price of share options outstanding (in dollars per share)	$ 50.36
Share options exercisable (in shares)	128
Weighted average remaining contractual life of share options exercisable (in years)	4 years 4 months 21 days
Weighted average exercise price of share options exercisable (in dollars per share)	$ 49.83

Weighted average remaining contractual term for option awards outstanding, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 718

-SubTopic 10

-Section 50

-Paragraph 2

-Subparagraph (e)(1)

-URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901

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- Definition

Weighted average exercise price as of the balance sheet date for those equity-based payment arrangements exercisable and outstanding.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

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-SubTopic 10

-Section 50

-Paragraph 2

-Subparagraph (c)(1)(iii)

-URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901

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- Definition

Weighted average remaining contractual term of exercisable stock options, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 718

-SubTopic 10

-Section 50

-Paragraph 2

-Subparagraph (e)(2)

-URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901

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- Definition

The floor of a customized range of exercise prices for purposes of disclosing shares potentially issuable under outstanding stock option awards on all stock option plans and other required information pertaining to awards in the customized range.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 718

-SubTopic 10

-Section 50

-Paragraph 2

-Subparagraph (g)

-URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901

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- Definition

The number of shares reserved for issuance pertaining to the outstanding exercisable stock options as of the balance sheet date in the customized range of exercise prices for which the market and performance vesting condition has been satisfied.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 718

-SubTopic 10

-Section 50

-Paragraph 2

-Subparagraph (c)(iii)

-URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901

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- Definition

The number of shares reserved for issuance pertaining to the outstanding stock options as of the balance sheet date for all option plans in the customized range of exercise prices.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 718

-SubTopic 10

-Section 50

-Paragraph 2

-Subparagraph (c)(1)(i)-(ii)

-URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901

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- Definition

The weighted average price as of the balance sheet date at which grantees could acquire the underlying shares with respect to all outstanding stock options which are in the customized range of exercise prices.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 718

-SubTopic 10

-Section 50

-Paragraph 2

-Subparagraph (c)(1)(i)-(ii)

-URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901

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- Definition

The ceiling of a customized range of exercise prices for purposes of disclosing shares potentially issuable under outstanding stock option awards on all stock option plans and other required information pertaining to awards in the customized range.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 718

-SubTopic 10

-Section 50

-Paragraph 2

-Subparagraph (g)

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