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Debt and liquidity (Senior Subordinated Notes) (Details) - USD ($) $ in Millions |  | 12 Months Ended
--- | --- | ---
Nov. 30, 2010 | Dec. 31, 2017
Debt Instrument [Line Items] |  |
Subordinated Borrowing, Interest Rate |  | 7.00%
Senior Subordinated Notes [Member] |  |
Debt Instrument [Line Items] |  |
Notes Issued | $ 200 |
Subordinated Borrowing, Interest Rate | 7.00% |

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- Definition The fair value of notes issued in noncash investing and financing activities. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 50 -Paragraph 5 -URI http://asc.fasb.org/extlink&oid=98513485&loc=d3e4332-108586 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 50 -Paragraph 3 -URI http://asc.fasb.org/extlink&oid=98513485&loc=d3e4304-108586 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 50 -Paragraph 4 -URI http://asc.fasb.org/extlink&oid=98513485&loc=d3e4313-108586 + Details
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