Allowance for Credit Losses (Changes in Allowance for Credit Losses) (Details) - USD ($)	3 Months Ended		6 Months Ended
	Jun. 30, 2023	Jun. 30, 2022	Jun. 30, 2023	Jun. 30, 2022
LOANS
Balance at Beginning of Period	$ 159,900,000	$ 136,300,000	$ 144,300,000	$ 138,400,000
Charge-Offs	(800,000)	0	(4,800,000)	(100,000)
Recoveries	800,000	5,500,000	1,900,000	8,800,000
Net Recoveries (Charge-Offs)	0	5,500,000	(2,900,000)	8,700,000
(Release of) Provision for Credit Losses	(7,400,000)	(3,600,000)	11,100,000	(8,900,000)
Balance at End of Period	152,500,000	138,200,000	152,500,000	138,200,000
UNDRAWN LOAN COMMITMENTS AND STANDBY LETTERS OF CREDIT
Balance at Beginning of Period	34,300,000	37,500,000	38,500,000	34,100,000
Charge-Offs	0	0	0	0
Recoveries	0	0	0	0
Net Recoveries (Charge-Offs)	0	0	0	0
(Release of) Provision for Credit Losses	(8,300,000)	6,000,000.0	(12,500,000)	9,400,000
Balance at End of Period	26,000,000.0	43,500,000	26,000,000.0	43,500,000
HELD TO MATURITY DEBT SECURITIES
Balance at Beginning of Period	16,000,000.0	14,600,000	16,000,000.0	11,200,000
Charge-Offs	0	0	0	0
Recoveries	0	0	0	0
Net Recoveries (Charge-Offs)	0	0	0	0
(Release of) Provision for Credit Losses	700,000	800,000	700,000	4,200,000
Balance at End of Period	16,700,000	15,400,000	16,700,000	15,400,000
OTHER FINANCIAL ASSETS
Balance at Beginning of Period	1,000,000.0	1,100,000	800,000	1,000,000.0
Charge-Offs	0	0	0	0
Recoveries	0	0	0	0
Net Recoveries (Charge-Offs)	0	0	0	0
(Release of) Provision for Credit Losses(1)	0	0	200,000	100,000
Balance at End of Period	1,000,000.0	1,100,000	1,000,000.0	1,100,000
TOTAL
Balance at Beginning of Period	211,200,000	189,500,000	199,600,000	184,700,000
Charge-Offs	(800,000)	0	(4,800,000)	(100,000)
Recoveries	800,000	5,500,000	1,900,000	8,800,000
Net Recoveries (Charge-Offs)	0	5,500,000	(2,900,000)	8,700,000
(Release of) Provision for Credit Losses	(15,000,000.0)	3,200,000	(500,000)	4,800,000
Balance at End of Period	196,200,000	198,200,000	196,200,000	198,200,000
Corporate Debt
HELD TO MATURITY DEBT SECURITIES
Balance at Beginning of Period	1,900,000	1,600,000	1,900,000	1,400,000
(Release of) Provision for Credit Losses	(500,000)	200,000	(500,000)	400,000
Balance at End of Period	1,400,000	1,800,000	1,400,000	1,800,000
TOTAL
(Release of) Provision for Credit Losses	$ (500,000)	$ 1,300,000	$ 0	$ 1,700,000