Investments - Debt and Short-term Investments (Details) - USD ($)	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2024	Sep. 30, 2023	Sep. 30, 2024	Sep. 30, 2023	Dec. 31, 2023
Investments
Other investments	$ 53,993,000		$ 53,993,000		$ 59,081,000
Investment in unconsolidated investees	65,271,000		65,271,000		56,966,000
Cash and Short-term Investments
Cash	60,634,000		60,634,000		36,424,000
Short-term investments	203,296,000		203,296,000		134,923,000
Other Unconsolidated Investees | Maximum
Investments
Investment in unconsolidated investees	1,000,000		1,000,000
Prime Holdings Insurance Services, Inc. (Prime)
Investments
Investment in unconsolidated investees	65,000,000		65,000,000
Dividends received from investments in unconsolidated investees			3,000,000		0
FHLBC
Investments
Investments pledged as collateral	$ 57,000,000		$ 57,000,000
Investment in federal home loan bank stock	us-gaap:AssetPledgedAsCollateralMember		us-gaap:AssetPledgedAsCollateralMember
Federal home loan bank borrowings fair value disclosure	$ 50,000,000		$ 50,000,000
Investment In Low Income Housing Tax Credit Partnerships (LIHTC)
Investments
Other investments	8,000,000		8,000,000		10,000,000
Amortization on low income housing investments	$ 1,000,000	$ 1,000,000	$ 2,000,000	$ 2,000,000
Amortization of tax credit partnerships, Statement of Income	Income tax expense	Income tax expense	Income tax expense	Income tax expense
Total tax benefit on investments in tax credit partnership	$ 1,000,000	$ 1,000,000	$ 2,000,000	$ 2,000,000
Investment In Low Income Housing Tax Credit Partnerships (LIHTC) | Maximum
Investments
Qualified affordable housing project investments, unfunded commitment	1,000,000		1,000,000
Historic tax credit partnerships (HTC)
Investments
Other investments	9,000,000		9,000,000		13,000,000
Amortization on qualifying equity investments	$ 1,000,000	$ 1,000,000	$ 3,000,000	$ 3,000,000
Amortization of tax credit partnerships, Statement of Income	Income tax expense	Income tax expense	Income tax expense	Income tax expense
Total tax benefit on investments in tax credit partnership	$ 2,000,000	$ 1,000,000	$ 4,000,000	$ 4,000,000
Investment in Private Funds
Investments
Other investments	25,000,000		25,000,000		$ 28,000,000
Fair value investments entities that calculate net asset value per share, unfunded commitments	$ 4,000,000		$ 4,000,000