Parent Company Financial Statements - Statements of Cash Flows of Parent Company (Detail) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
CASH FLOWS FROM OPERATING ACTIVITIES:
Net income	$ 21,860	$ 20,046	$ 19,039
Adjustments to reconcile net income to net cash provided by operating activities
Depreciation and amortization	2,937	5,358	6,445
Increase in other assets	(2,849)	(5,693)	(3,113)
Net cash provided by operating activities	22,387	20,996	25,118
CASH FLOWS FROM FINANCING ACTIVITIES:
Net proceeds from the exercise of stock options	361	43	304
Cash dividends paid	(2,196)	(2,191)	(2,186)
Net cash provided by financing activities	158,193	351,668	317,678
Net (decrease) increase in cash and cash equivalents	210,679	(57,605)	(55,483)
Cash and cash equivalents at beginning of year	94,678	152,283	207,766
Cash and cash equivalents at end of year	305,357	94,678	152,283
Century Bancorp, Inc. [Member]
CASH FLOWS FROM OPERATING ACTIVITIES:
Net income	21,860	20,046	19,039
Adjustments to reconcile net income to net cash provided by operating activities
Undistributed income of subsidiary	(23,470)	(21,701)	(20,684)
Depreciation and amortization	12	12	12
Increase in other assets	(1,067)	(3,500)	(416)
Decrease in liabilities	(71)
Net cash provided by operating activities	(2,736)	(5,143)	(2,049)
CASH FLOWS FROM FINANCING ACTIVITIES:
Net proceeds from the exercise of stock options	361	43	304
Cash dividends paid	(2,196)	(2,191)	(2,186)
Net cash provided by financing activities	(1,835)	(2,148)	(1,882)
Net (decrease) increase in cash and cash equivalents	(4,571)	(7,291)	(3,931)
Cash and cash equivalents at beginning of year	12,245	19,536	23,467
Cash and cash equivalents at end of year	$ 7,674	$ 12,245	$ 19,536