| **Stockholders' equity (deficit) (Details) (USD $)In Millions, except Share data, unless otherwise specified** | 6 Months Ended | 1 Months Ended |        | 1 Months Ended | 3 Months Ended | 6 Months Ended |
|----------------------------------------------------------------------------------------------------------------|----------------|----------------|--------|----------------|----------------|----------------|
||
| **Class of Stock [Line Items]**                                                                                |           |           |   |           |           |           |   |   |   |   |   |   |   |
| Stock Repurchase Program, Authorized Amount                                                                    |           |           | $ 175  |           | $ 100          |           |   |   |   |   |   |   |   |
| Payments for Repurchase of Common Stock                                                                        | 75             | 0              |   |           | 100            |           |   |   |   |   |   |   |   |
| Treasury Stock, Shares, Acquired                                                                               |           |           |   | 2,542,609      |           |           | 2,380,952 |   |   | 1,905,600 |   |   |   |
| Accelerated Share Repurchases, Settlement Payment or Receipt                                                   |           |           |   |           |           | 80             |   | 20 |   |   |   |   |   |
| Accelerated Share Repurchases, Initial Price Paid Per Share                                                    |           |           |   |           |           | $ 33.60        |   |   |   |   |   |   |   |
| Stock Repurchase Program, Remaining Authorized Repurchase Amount                                               |           |           |   |           | $ 20           |           |   |   |   |   | $ 75 | $ 70 | $ 5 |
| Treasury Stock, Additional Shares Acquired                                                                     |           |           |   |           |           |           |   |   | 161,657 |   |   |   |   |

| X                                                                                                                                                                                                                                                               |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Treasury Stock, Additional Shares Acquired
+ References
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                    |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
The price paid per share to immediately purchase the targeted number of shares on the date of executing the accelerated share repurchase agreement.
+ References
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                    |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Dollar amount of the cash payment to or cash received from the bank; or the stock issuance to (stock received from) the bank in the settlement of the accelerated share repurchase agreement.
+ References
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| X                                                                                                                                                         |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------|
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|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
The cash outflow to reacquire common stock during the period.
+ References
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| X                                                                                                                                                                                                                                                                                                                          |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
The amount authorized by an entity's Board of Directors under a stock repurchase plan.
+ References
No definition available.
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| X                                                                                                                                                                                                                                                                                                                                                                  |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
The remaining repurchase amount authorized by an entity's Board of Directors under a stock repurchase plan.
+ References
No definition available.
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                        |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Number of shares that have been repurchased during the period and are being held in treasury.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 505 -SubTopic 10 -Section 50 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=6928386&loc=d3e21463-112644Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher AICPA -Name Accounting Principles Board Opinion (APB) -Number 12 -Paragraph 10 -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 SEC -Name Regulation S-X (SX) -Number 210 -Section 04 -Article 3Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 02 -Paragraph 29, 30 -Article 5
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