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Stockholders' Equity Stockholders' Equity (Tables) | 3 Months Ended
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Mar. 31, 2019
Stockholders' Equity Attributable to Parent [Abstract] |
Quarterly dividends declared [Table Text Block] | The Board declared quarterly dividends as follows:
(amounts per share) | 2019 |  |  | 2018 |
First quarter | $ | 0.30 |  |  | $ | 0.20 |

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- Definition Quarterly dividends declared [Table Text Block] - tabular disclosure of all or some of the information related to dividends declared and paid as of the financial reporting date. + References No definition available. + Details
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Namespace Prefix: | nsp_
Data Type: | nonnum:textBlockItemType
Balance Type: | na
Period Type: | duration

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- References No definition available. + Details
Name: | us-gaap_StockholdersEquityAbstract
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration