Aggregate dividends declared during the period for each share of common stock outstanding.

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

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 04

-Article 3

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 505

-SubTopic 10

-Section S99

-Paragraph 1

-Subparagraph (SX 210.3-04)

-URI http://asc.fasb.org/extlink&oid=27012166&loc=d3e187085-122770

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- Definition

The cash outflow for debt initially having maturity due after one year or beyond the normal operating cycle, if longer.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 230

-SubTopic 10

-Section 45

-Paragraph 15

-Subparagraph (b)

-URI http://asc.fasb.org/extlink&oid=31042434&loc=d3e3291-108585

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Glossary Financing Activities

-URI http://asc.fasb.org/extlink&oid=6513228

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Amount of a stock repurchase plan authorized by an entity's Board of Directors.

+ References

No definition available.

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Subsequent events (Details) (USD $)	0 Months Ended	12 Months Ended			0 Months Ended
Subsequent events (Details) (USD $)	Feb. 03, 2015	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Feb. 11, 2015
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Repayment of long-term debt		$ 375,000,000us-gaap_RepaymentsOfLongTermDebt	$ 6,000,000us-gaap_RepaymentsOfLongTermDebt	$ 0us-gaap_RepaymentsOfLongTermDebt
Dividends per common share (in dollars per share)	$ 0.23	$ 0.20	$ 0.19	$ 0.18
Subsequent Events
Subsequent Event [Line Items]
Stock repurchase program, dollar amount authorized	750,000,000
Repayment of long-term debt					$ 250,000,000
Dividends per common share (in dollars per share)	$ 0.23