Form N-CSR Cover
| Form N-CSR Cover | 6 Months Ended |
| Jun. 30, 2024 | |
| Shareholder Report [Line Items] | |
| Document Type | N-CSRS |
| Amendment Flag | false |
| Registrant Name | JOHN HANCOCK COLLATERAL TRUST |
| Entity Central Index Key | 0001631942 |
| Document Period End Date | Jun. 30, 2024 |
Shareholder Report
| Shareholder Report | 6 Months Ended |
| Jun. 30, 2024 USD ($) Holding | |
| Shareholder Report [Line Items] | |
| Document Type | N-CSRS |
| Amendment Flag | false |
| Registrant Name | JOHN HANCOCK COLLATERAL TRUST |
| Entity Central Index Key | 0001631942 |
| Document Period End Date | Jun. 30, 2024 |
| C000154149 [Member] | |
| Shareholder Report [Line Items] | |
| Fund Name | John Hancock Collateral Trust |
| Class Name | John Hancock Collateral Trust |
| No Trading Symbol Flag | true |
| Annual or Semi-Annual Statement [Text Block] | This semi-annual shareholder report contains important information about the John Hancock Collateral Trust (the fund) for the period of January 1, 2024 to June 30, 2024. |
| Shareholder Report Annual or Semi-Annual | semi-annual shareholder report |
| Additional Information [Text Block] | You can find additional information about the fund at jhinvestments.com/collateral-trust |
| Additional Information Phone Number | 800-225-5291 |
| Additional Information Website | jhinvestments.com/collateral-trust |
| Expenses [Text Block] | What were the fund costs during the last six months? (Based on a hypothetical $10,000 investment) Fund Costs of a $10,000 investment Costs paid as a percentage Collateral Trust $ 0.10 % |
| Expenses Paid, Amount | $ 5 |
| Expense Ratio, Percent | 0.10% |
| Net Assets | $ 1,945,627,192 |
| Holdings Count | Holding | 127 |
| Additional Fund Statistics [Text Block] | Fund Statistics Fund net assets $ Total number of portfolio holdings $ Weighted Average Maturity 47 Days |
| Holdings [Text Block] | Graphical Representation of Holdings The tables below show the investment makeup of the fund, representing percentage of the total net assets of the fund. Portfolio Composition Commercial paper 27.8 % U.S. Government 27.0 % Corporate interest-bearing obligations 16.4 % Time deposits 8.4 % Certificate of deposit 7.8 % Repurchase agreement 7.6 % U.S. Government Agency 4.7 % Other assets and liabilities, net 0.3 % Maturity Composition Holdings may not have been held by the fund for the entire period and are subject to change without notice. Portfolio composition is subject to review in accordance with the fund’s investment strategy and may vary in the future. Current and future portfolio holdings are subject to risk and may change at any time. |
Shareholder Report, Holdings (D
| Shareholder Report, Holdings (Details) - C000154149 [Member] | Jun. 30, 2024 |
| Zero To 30 days [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 66.90% |
| Thirty One To 60 days [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 7.20% |
| Sixty One To 90 days [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 13.40% |
| Over 90 days [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 12.50% |
| Bank Time Deposits [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 8.40% |
| Certificates of Deposit [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 7.80% |
| Commercial Paper [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 27.80% |
| Repurchase Agreements [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 7.60% |
| US Government Corporations and Agencies Securities [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 4.70% |
| US Government Debt Securities [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 27% |
| Corporate interest bearing obligations [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 16.40% |
| Other assets and liabilities net [Member] | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 0.30% |