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Stockholders' Equity - The Company's and the Bank's Actual Capital Amounts and Ratios (Detail) (USD $)
In Thousands, unless otherwise specified
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Dec. 31, 2013
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Dec. 31, 2012
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Meridian Interstate Bancorp, Inc/Parent Company [Member]
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| Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items] |
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| Total Capital (to Risk Weighted Assets) |
$ 259,577 |
$ 237,527 |
| Total Capital (to Risk Weighted Assets), Ratio |
10.80% |
11.90% |
| Total Capital (to Risk Weighted Assets), Minimum Capital Requirement |
192,797 |
159,344 |
| Total Capital (to Risk Weighted Assets), Minimum Capital Requirement, Ratio |
8.00% |
8.00% |
| Total Capital (to Risk Weighted Assets), Minimum to Be Well Capitalized Under Prompt Corrective Action Provisions |
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| Total capital (to Risk Weighted Assets), Minimum to Be Well Capitalized Under Prompt Corrective Action Provisions, Ratio |
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| Tier 1 Capital (to Risk Weighted Assets) |
231,342 |
215,255 |
| Tier 1 Capital (to Risk Weighted Assets), Ratio |
9.60% |
10.80% |
| Tier 1 Capital (to Risk Weighted Assets), Minimum Capital Requirement |
96,398 |
79,672 |
| Tier 1 Capital (to Risk Weighted Assets), Minimum Capital Requirement, Ratio |
4.00% |
4.00% |
| Tier 1 Capital (to Risk Weighted Assets) , Minimum to Be Well Capitalized Under Prompt Corrective Action Provisions |
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| Tier 1 Capital (to Risk Weighted Assets) , Minimum to Be Well Capitalized Under Prompt Corrective Action Provisions, Ratio |
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| Tier 1 Capital (to Average Assets) |
231,342 |
215,255 |
| Tier 1 Capital (to Average Assets), Ratio |
8.70% |
9.70% |
| Tier 1 Capital (to Average Assets), Minimum Capital Requirement |
105,999 |
88,858 |
| Tier 1 Capital (to Average Assets), Minimum Capital Requirement, Ratio |
4.00% |
4.00% |
| Tier 1 capital (to Average Assets), Minimum to Be Well Capitalized Under Prompt Corrective Action Provisions |
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| Tier 1 capital (to Average Assets), Minimum to Be Well Capitalized Under Prompt Corrective Action Provisions, Ratio |
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Bank [Member]
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| Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items] |
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| Total Capital (to Risk Weighted Assets) |
246,100 |
201,113 |
| Total Capital (to Risk Weighted Assets), Ratio |
10.20% |
10.20% |
| Total Capital (to Risk Weighted Assets), Minimum Capital Requirement |
192,511 |
157,224 |
| Total Capital (to Risk Weighted Assets), Minimum Capital Requirement, Ratio |
8.00% |
8.00% |
| Total Capital (to Risk Weighted Assets), Minimum to Be Well Capitalized Under Prompt Corrective Action Provisions |
240,639 |
196,531 |
| Total capital (to Risk Weighted Assets), Minimum to Be Well Capitalized Under Prompt Corrective Action Provisions, Ratio |
10.00% |
10.00% |
| Tier 1 Capital (to Risk Weighted Assets) |
217,865 |
178,852 |
| Tier 1 Capital (to Risk Weighted Assets), Ratio |
9.10% |
9.10% |
| Tier 1 Capital (to Risk Weighted Assets), Minimum Capital Requirement |
96,256 |
78,612 |
| Tier 1 Capital (to Risk Weighted Assets), Minimum Capital Requirement, Ratio |
4.00% |
4.00% |
| Tier 1 Capital (to Risk Weighted Assets) , Minimum to Be Well Capitalized Under Prompt Corrective Action Provisions |
144,383 |
117,918 |
| Tier 1 Capital (to Risk Weighted Assets) , Minimum to Be Well Capitalized Under Prompt Corrective Action Provisions, Ratio |
6.00% |
6.00% |
| Tier 1 Capital (to Average Assets) |
217,865 |
178,852 |
| Tier 1 Capital (to Average Assets), Ratio |
8.20% |
8.20% |
| Tier 1 Capital (to Average Assets), Minimum Capital Requirement |
105,702 |
87,742 |
| Tier 1 Capital (to Average Assets), Minimum Capital Requirement, Ratio |
4.00% |
4.00% |
| Tier 1 capital (to Average Assets), Minimum to Be Well Capitalized Under Prompt Corrective Action Provisions |
$ 132,127 |
$ 109,678 |
| Tier 1 capital (to Average Assets), Minimum to Be Well Capitalized Under Prompt Corrective Action Provisions, Ratio |
5.00% |
5.00% |