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Stockholders' Equity (Details) (USD $) In Thousands, except Share data, unless otherwise specified | 3 Months Ended | 12 Months Ended
--- | --- | ---
Dec. 31, 2014 | Sep. 30, 2014 | Jun. 30, 2014 | Mar. 31, 2014 | Dec. 31, 2013 | Sep. 30, 2013 | Jun. 30, 2013 | Mar. 31, 2013 | Dec. 31, 2014 | Dec. 31, 2013 | Dec. 31, 2012
Stockholders' Equity Note [Abstract] |  |  |  |  |  |  |  |  |  |  |  |
Authorized number of shares to be repurchased (in shares) | 15,500,000 us-gaap_StockRepurchaseProgramNumberOfSharesAuthorizedToBeRepurchased |  |  |  |  |  |  |  |  | 15,500,000 us-gaap_StockRepurchaseProgramNumberOfSharesAuthorizedToBeRepurchased |  |
Repurchase of treasury stock (in shares) |  |  |  |  |  |  |  |  |  | 580,804 us-gaap_TreasuryStockSharesAcquired | 479,903 us-gaap_TreasuryStockSharesAcquired |
Shares withheld for tax withholding obligations for the vesting of restricted stock awards (in shares) |  |  |  |  |  |  |  |  |  | 112,458 us-gaap_SharesPaidForTaxWithholdingForShareBasedCompensation | 116,931 us-gaap_SharesPaidForTaxWithholdingForShareBasedCompensation |
Remaining number of shares authorized to be repurchased (in shares) | 768,765 us-gaap_StockRepurchaseProgramRemainingNumberOfSharesAuthorizedToBeRepurchased |  |  |  |  |  |  |  |  | 768,765 us-gaap_StockRepurchaseProgramRemainingNumberOfSharesAuthorizedToBeRepurchased |  |
Payments of Dividends [Abstract] |  |  |  |  |  |  |  |  |  |  |  |
Dividends declared per share of common stock (in dollars per share) | $ 0.19 us-gaap_CommonStockDividendsPerShareDeclared |  | $ 0.19 us-gaap_CommonStockDividendsPerShareDeclared | $ 0.19 us-gaap_CommonStockDividendsPerShareDeclared | $ 0.17 us-gaap_CommonStockDividendsPerShareDeclared | $ 0.17 us-gaap_CommonStockDividendsPerShareDeclared | $ 0.17 us-gaap_CommonStockDividendsPerShareDeclared | $ 0.17 us-gaap_CommonStockDividendsPerShareDeclared | $ 0.17 us-gaap_CommonStockDividendsPerShareDeclared |  |  |
Special dividends declared, per share of commong stock (in dollars per share) | $ 2.00 nsp_Specialdividendpershare |  |  |  |  |  |  |  |  |  |  |
Total regular and special quarterly dIvidend declared, per share of common stock (in dollars per share) | $ 2.19 nsp_TotalQuarterlyDIvidendRegularandSpecial | [1] |  |  |  |  |  |  |  |  |  |
Payments of Ordinary Dividends, Common Stock |  |  |  |  |  |  |  |  |  | $ 69,493 us-gaap_PaymentsOfDividendsCommonStock | $ 17,386 us-gaap_PaymentsOfDividendsCommonStock | $ 42,728 us-gaap_PaymentsOfDividendsCommonStock
Special dividend |  |  |  |  |  |  |  |  |  | 50,664 nsp_SpecialDividend |  | 25,700 nsp_SpecialDividend
Total Dividends Paid, Ordinary and Special |  |  |  |  |  |  |  |  |  | $ 69,493 nsp_TotalDividendsPaidOrdinaryandSpecial |  |
Class of Stock [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Preferred stock, authorized (in shares) | 20,000,000 us-gaap_PreferredStockSharesAuthorized |  |  |  |  | 20,000,000 us-gaap_PreferredStockSharesAuthorized |  |  |  | 20,000,000 us-gaap_PreferredStockSharesAuthorized | 20,000,000 us-gaap_PreferredStockSharesAuthorized |
Common stock purchase right per preferred stock share (in shares) |  |  |  |  |  |  |  |  |  | 1 nsp_CommonStockPurchaseRightPerPreferredStockShare |  |
Series A Junior Participating Preferred Stock [Member] |  |  |  |  |  |  |  |  |  |  |  |
Class of Stock [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Preferred stock, authorized (in shares) | 600,000 us-gaap_PreferredStockSharesAuthorized / us-gaap_StatementClassOfStockAxis = nsp_SeriesJuniorParticipatingPreferredStockMember |  |  |  |  |  |  |  |  | 600,000 us-gaap_PreferredStockSharesAuthorized / us-gaap_StatementClassOfStockAxis = nsp_SeriesJuniorParticipatingPreferredStockMember |  |
[1] | 1) Includes a $2.00 per share special dividend

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- Definition Amount of cash outflow in the form of special dividends (not ordinary) to common shareholders of the parent entity. + References No definition available. + Details
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- Definition Per share amount of special dividend declared. + References No definition available. + Details
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- Definition The total quarterly dividend declared, both regular and special. + References No definition available. + Details
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- Definition Aggregate dividends declared during the period for each share of common stock outstanding. + References 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 + Details
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- Definition Amount of cash outflow in the form of ordinary dividends to common shareholders of the parent entity. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Glossary Financing Activities -URI http://asc.fasb.org/extlink&oid=6513228 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 15 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=31042434&loc=d3e3291-108585 + Details
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