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Debt and liquidity debt and liquidity (Senior Notes) (Details) - USD ($) | Dec. 31, 2017 | Sep. 30, 2017 | Nov. 20, 2012
--- | --- | --- | ---
Debt Instrument [Line Items] |  |  |
Debt Instrument, Interest Rate, Stated Percentage | 6.375% |  |
Senior Notes |  |  |
Debt Instrument [Line Items] |  |  |
Debt Instrument, Face Amount |  |  | $ 300,000,000
Debt Instrument, Interest Rate, Stated Percentage | 6.375% |  | 6.375%
Redemption price percentage of principal prior to November 15, 2016 |  |  | 100.00%
Repurchase Percentage Price Of Aggregate Principal Due To Change In Control |  |  | 101.00%
Minimum |  |  |
Debt Instrument [Line Items] |  |  |
Unpaid indebtedness after maturity or acceleration | $ 50,000,000 |  |
Unpaid judgment or decree | $ 50,000,000 |  |
Minimum | Senior Notes |  |  |
Debt Instrument [Line Items] |  |  |
Unpaid judgment or decree |  | $ 50,000,000 |

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- Definition Face (par) amount of debt instrument at time of issuance. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 835 -SubTopic 30 -Section 55 -Paragraph 8 -URI http://asc.fasb.org/extlink&oid=64930739&loc=d3e28878-108400 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 835 -SubTopic 30 -Section 45 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=64930785&loc=d3e28551-108399 + Details
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- Definition Contractual interest rate for funds borrowed, under the debt agreement. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.22(a)(1)) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 + Details
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