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Borrowings - Narrative (Details) - USD ($)		1 Months Ended	12 Months Ended		69 Months Ended	119 Months Ended
Borrowings - Narrative (Details) - USD ($)	Feb. 15, 2024	May 31, 2023	Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2024	Jan. 16, 2024	Oct. 30, 2020	Dec. 16, 2019	Mar. 25, 2019	Feb. 11, 2014	Sep. 17, 2003
Debt Instrument [Line Items]
Market value pledged to repurchase agreements			$ 358,400,000	$ 381,400,000	$ 358,400,000
Notes and subordinated debt			$ 899,071,000	1,036,974,000	$ 899,071,000
Senior Notes Due 2024 \| Senior Notes
Debt Instrument [Line Items]
Notes and subordinated debt										$ 150,000,000.0
Debt instrument, stated interest rate, percentage										4.375%
Amount repurchased and retired		$ 17,500,000
Repurchased and retired in cents on the dollar		$ 0.96
Gain recognized upon extinguishment				$ 700,000
Repayment of long-term debt	$ 132,600,000
Senior Notes Due 2024 \| Senior Notes \| US Treasury (UST) Interest Rate
Debt Instrument [Line Items]
Basis spread						0.30%
Senior Notes Due 2029 \| Senior Notes
Debt Instrument [Line Items]
Notes and subordinated debt									$ 300,000,000.0
Debt instrument, stated interest rate, percentage									4.10%
Senior Notes Due 2029 \| Senior Notes \| US Treasury (UST) Interest Rate
Debt Instrument [Line Items]
Basis spread					0.25%
2029 subordinated notes \| Sterling
Debt Instrument [Line Items]
Aggregate principal amount								$ 274,000,000
Fixed-to-floating rate								4.00%
Interest rate on notes			6.84%		6.84%
2029 subordinated notes \| Subordinated Debt \| Secured Overnight Financing Rate (SOFR)
Debt Instrument [Line Items]
Basis spread			2.53%
2030 subordinated notes \| Sterling
Debt Instrument [Line Items]
Aggregate principal amount							$ 225,000,000.0
Fixed-to-floating rate							3.875%
2030 subordinated notes \| Subordinated Debt \| Secured Overnight Financing Rate (SOFR)
Debt Instrument [Line Items]
Basis spread			3.69%
2033 Junior Subordinated Notes \| Junior Subordinated Debt
Debt Instrument [Line Items]
Notes and subordinated debt											$ 77,300,000
2033 Junior Subordinated Notes \| Junior Subordinated Debt \| Secured Overnight Financing Rate (SOFR)
Debt Instrument [Line Items]
Interest rate on notes			7.56%	8.59%	7.56%
Interest rate spread of LIBOR plus (as a percent)							2.95%