Condensed Financial Statements of Parent Company - Statements of Income Pertaining only to Meridian Interstate Bancorp (Detail) (USD $)
In Thousands, unless otherwise specified
 0 Months Ended 3 Months Ended 12 Months Ended
Jun. 08, 2012
Dec. 31, 2013
Sep. 30, 2013
Jun. 30, 2013
Mar. 31, 2013
Dec. 31, 2012
Sep. 30, 2012
Jun. 30, 2012
Mar. 31, 2012
Dec. 31, 2013
Dec. 31, 2012
Dec. 31, 2011
Income:                        
Gain on sales of securities, net                   $ 9,636 $ 5,568 $ 4,464
Equity income on investment in affiliate bank                      310 1,110
Gain on sale of investment in affiliate bank 4,800             4,800      4,819   
Total interest and dividend income   25,213 24,320 23,255 22,416 22,183 21,181 20,968 20,637 95,204 84,969 78,812
Applicable income tax provision   2,232 3,272 1,200 1,367 1,079 1,554 2,639 1,058 8,071 6,330 6,601
Net income   4,015 5,321 3,025 3,068 2,109 2,714 5,447 2,156 15,429 12,426 11,970
Meridian Interstate Bancorp, Inc/Parent Company [Member]
                        
Income:                        
Interest and dividend income                   260 361 220
Gain on sales of securities, net                   940 364  
Equity income on investment in affiliate bank                     310 1,110
Gain on sale of investment in affiliate bank                     4,819  
Total interest and dividend income                   1,200 5,854 1,330
Operating expenses                   958 880 800
Income before income taxes and equity in undistributed earnings of subsidiaries                   242 4,974 530
Applicable income tax provision                   63 1,986 210
Income after income tax provision, Total                   179 2,988 320
Equity in undistributed earnings of subsidiaries                   15,250 9,438 11,650
Net income                   $ 15,429 $ 12,426 $ 11,970