Investment Securities - Narrative (Details)	9 Months Ended		12 Months Ended
	Sep. 30, 2021 USD ($) security	Sep. 30, 2020 USD ($) security	Dec. 31, 2020 USD ($) security
Investment Securities
Investment securities available-for-sale, allowance	$ 0
Investment securities available-for-sale	732,020,000		$ 745,822,000
Debt securities, available-for-sale, accumulated gross unrealized gain (loss)	3,500,000		21,300,000
Debt securities, available-for-sale, unrealized loss position	$ 319,571,000		$ 86,894,000
Percentage of securities in unrealized loss positions, available-for-sale securities	43.70%		11.70%
Debt securities, available-for-sale, unrealized loss position, accumulated loss	$ 7,900,000		$ 731,000
Equity securities held-for-trading	0		526,000
Accrued interest receivable	$ 2,800,000		2,600,000
Number of days past due to be placed in nonaccrual status	90 days
Purchases of securities available-for-sale	$ 153,153,000	$ 488,806,000
Estimated fair value of debt securities have right to call or prepay the obligations	67,500,000		90,800,000
Estimated fair value of debt securities that have right to call or prepay the obligations, scheduled maturities of less than one year	3,000,000
Estimated fair value of debt securities have right to call or prepay the obligations, scheduled maturities of after one year through five years	9,600,000		12,900,000
Estimated fair value of debt securities have right to call or prepay the obligations, scheduled maturities after six years through ten years	45,100,000		68,100,000
Estimated fair value of debt securities have right to call or prepay the obligations, scheduled maturities of which is after 10 years	9,800,000		9,800,000
Proceeds from sales of investment securities available-for-sale and equity securities held-for-trading	520,000	143,315,000
US Government Sponsored Enterprises Debt Securities Excluding Specified Securities
Investment Securities
Investment securities available-for-sale	4,600,000		7,300,000
GSE debentures
Investment Securities
Investment securities available-for-sale	233,286,000		278,645,000
Debt securities, available-for-sale, accumulated gross unrealized gain (loss)	$ (800,000)		$ 4,800,000
AFS number of positions | security	49		54
Number of securities in unrealized loss positions | security	12		7
Purchases of securities available-for-sale		$ 208,900,000
Number of securities transferred | security		9
Amount of transfer		$ 25,500,000
GSE CMOs
Investment Securities
Investment securities available-for-sale	$ 31,800,000		$ 46,000,000
Debt securities, available-for-sale, accumulated gross unrealized gain (loss)	$ 600,000		$ 1,100,000
AFS number of positions | security	33		33
Number of securities in unrealized loss positions | security	2		2
Purchases of securities available-for-sale	$ 0	0
GSE MBSs
Investment Securities
Investment securities available-for-sale	221,800,000		$ 323,600,000
Debt securities, available-for-sale, accumulated gross unrealized gain (loss)	$ 4,500,000		$ 11,000,000
AFS number of positions | security	139		140
Number of securities in unrealized loss positions | security	20		17
Purchases of securities available-for-sale		$ 258,700,000
Number of securities transferred | security		8
Amount of transfer		$ 9,000,000
Corporate debt obligations
Investment Securities
Investment securities available-for-sale	$ 22,969,000		$ 23,467,000
Debt securities, available-for-sale, accumulated gross unrealized gain (loss)	$ 800,000		$ 1,200,000
AFS number of positions | security	6		6
Number of securities in unrealized loss positions | security	0		0
Purchases of securities available-for-sale	$ 0	$ 0
Number of securities transferred | security		1
Amount of transfer		$ 500,000
U.S. Treasury bonds
Investment Securities
Investment securities available-for-sale	221,600,000		$ 73,600,000
Debt securities, available-for-sale, accumulated gross unrealized gain (loss)	$ (1,700,000)		$ 3,200,000
AFS number of positions | security	27		12
Number of securities in unrealized loss positions | security	16		1
Purchases of securities available-for-sale	$ 153,200,000	21,200,000
Municipal obligations
Investment Securities
Purchases of securities available-for-sale	$ 0	$ 0
Number of securities transferred | security	0	93	0
Amount of transfer		$ 47,700,000
Foreign government obligations
Investment Securities
Investment securities available-for-sale	$ 498,000		$ 496,000
AFS number of positions | security	1		1
Purchases of securities available-for-sale	$ 0	$ 0
Collateral Pledged
Investment Securities
Investment securities pledged as collateral	426,100,000		$ 448,700,000
Outstanding borrowings	$ 0		$ 0