Stock-based Compensation Plans - Grant Activities (Details) (2010 LTIP [Member], USD $)	6 Months Ended
Jun. 30, 2013	Jun. 30, 2012
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Award vesting period	3 years
Restricted stock units [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Granted, number	87,268	81,468
Grants in period, weighted average grant date fair value	$ 49.38	$ 39.67
Performance shares [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Granted, number	90,500	85,252
Grants in period, weighted average grant date fair value	$ 49.13	$ 39.59
Twelve months following date of grant [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares cliff vest percentage	33.33%
January 2015 [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares cliff vest percentage	33.33%
January 2016 [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares cliff vest percentage	33.33%

Share-based Compensation Arrangement by Share-based Payment Award, Cliff Vest Percentage

No definition available.

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- Definition

Period which an employee's right to exercise an award is no longer contingent on satisfaction of either a service condition, market condition or a performance condition, 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 (a)(1)

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

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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 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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- Details

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- Definition

Net number of non-option equity instruments granted to participants.

+ 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)(iv)(1)

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

+ Details

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