Borrowings - Schedule of FHLB Advances (Details) - USD ($) $ in Thousands	Jun. 30, 2026	Dec. 31, 2025
Total Outstanding
Maturing within 1 year	$ 3,650,000	$ 2,970,000
After 1 year but within 2 years	386	201
After 2 years but within 3 years	0	201
After 3 years but within 4 years	601	615
After 4 years but within 5 years	4,332	3,669
After 5 years	5,927	6,032
Total FHLB advances	$ 3,661,246	$ 2,980,718
Weighted-Average Contractual Coupon Rate
Maturing within 1 year	3.85%	3.44%
After 1 year but within 2 years	1.38%	0.00%
After 2 years but within 3 years	0.00%	2.75%
After 3 years but within 4 years	1.75%	1.75%
After 4 years but within 5 years	1.04%	1.25%
After 5 years	2.16%	2.16%
Total FHLB advances	3.84%	3.44%
Aggregate market value of assets pledged as collateral	$ 22,906,954	$ 16,331,016
Remaining borrowing capacity at the FHLB of Boston	$ 12,955,009	$ 7,882,187

Amount of assets pledged to secure a debt instrument.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Name Accounting Standards Codification
-Topic 942
-SubTopic 470
-Section 50
-Paragraph 3
-Subparagraph (d)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147477734/942-470-50-3

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Amount of advance from Federal Home Loan Bank (FHLBank).

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 942
-SubTopic 470
-Name Accounting Standards Codification
-Section 45
-Paragraph 1
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147478878/942-470-45-1

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Amount of advances or credit lines available from the Federal Home Loan Bank (FHLB).

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Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 942
-SubTopic 210
-Name Accounting Standards Codification
-Section S45
-Paragraph 1
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147477657/942-210-S45-1

Reference 2: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 942
-SubTopic 210
-Name Accounting Standards Codification
-Section S50
-Paragraph 1
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147478478/942-210-S50-1

Reference 3: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 942
-SubTopic 210
-Name Accounting Standards Codification
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03(13)(3)(a))
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147478546/942-210-S99-1

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-Section 45
-Paragraph 1
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147478878/942-470-45-1

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Amount of advances from Federal Home Loan Bank (FHLBank) maturing after the fifth rolling twelve months following the latest balance sheet. For interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 470
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 1
-Publisher FASB
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Amount of advances from Federal Home Loan Bank (FHLBank) maturing in the next rolling twelve months following the latest balance sheet. For interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date.

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Amount of advances from Federal Home Loan Bank (FHLBank) maturing in the fifth rolling twelve months following the latest balance sheet. For interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date.

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Amount of advances from Federal Home Loan Bank (FHLBank) maturing in the fourth rolling twelve months following the latest balance sheet. For interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date.

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Amount of advances from Federal Home Loan Bank (FHLBank) maturing in the third rolling twelve months following the latest balance sheet. For interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date.

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Amount of advances from Federal Home Loan Bank (FHLBank) maturing in the second rolling twelve months following the latest balance sheet. For interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date.

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Weighted average interest rate on advances made and reported by Federal Home Loan Bank (FHLBank).

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Name Regulation S-K (SK)
-Number 229
-Section 1404
-Paragraph b
-Subparagraph (2)
-Publisher SEC

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Weighted average interest rate on advances made and reported by Federal Home Loan Bank (FHLBank) maturing after the fifth rolling twelve months following the latest balance sheet. For interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date.

+ References