Investment Securities Held-to-Maturity - Unrealized Market Loss of Securities (Detail) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020
Schedule of Held-to-maturity Securities [Line Items]
Less Than 12 Months, Fair Value	$ 1,780,028	$ 465,271
Less Than 12 Months, Unrealized Losses	31,002	1,898
12 Months or Longer, Fair Value	254,275	0
12 Months or Longer, Unrealized Losses	6,143	0
Total, Fair Value	2,034,303	465,271
Total, Unrealized Losses	37,145	1,898
US Government Sponsored Enterprises Mortgage-Backed Securities [Member]
Schedule of Held-to-maturity Securities [Line Items]
Less Than 12 Months, Fair Value	194,263	162,870
Less Than 12 Months, Unrealized Losses	2,579	866
12 Months or Longer, Fair Value	146,298	0
12 Months or Longer, Unrealized Losses	2,444	0
Total, Fair Value	340,561	162,870
Total, Unrealized Losses	5,023	866
SBA Backed Securities [Member]
Schedule of Held-to-maturity Securities [Line Items]
Less Than 12 Months, Fair Value	0	0
Less Than 12 Months, Unrealized Losses	0	0
12 Months or Longer, Fair Value	0	0
12 Months or Longer, Unrealized Losses	0	0
Total, Fair Value	0	0
Total, Unrealized Losses	0	0
U.S. Government Agency and Sponsored Enterprises Mortgage-Backed Securities [Member]
Schedule of Held-to-maturity Securities [Line Items]
Less Than 12 Months, Fair Value	1,585,765	302,401
Less Than 12 Months, Unrealized Losses	28,423	1,032
12 Months or Longer, Fair Value	107,977	0
12 Months or Longer, Unrealized Losses	3,699	0
Total, Fair Value	1,693,742	302,401
Total, Unrealized Losses	$ 32,122	$ 1,032

CENTURY BANCORP INC 10-Q