Stock-based Compensation Plans - Grant Activities (Details) (USD $)	9 Months Ended	1 Months Ended
Sep. 30, 2013 Restricted stock units [Member]	Sep. 30, 2012 Restricted stock units [Member]	Feb. 28, 2013 Performance shares [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Granted, number	87,268	81,468	90,500
Grants in period, weighted average grant date fair value	$ 49.38	$ 39.67	$ 49.13

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).

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

Name:	us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue
Namespace Prefix:	us-gaap_
Data Type:	num:perShareItemType
Balance Type:	na
Period Type:	duration