Allowance for Credit Losses (Changes in Allowance for Credit Losses) (Details) - USD ($)	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
LOANS AND LEASES
Balance at Beginning of Period	$ 147,200,000	$ 114,500,000	$ 104,500,000	$ 112,600,000
Charge-Offs	(400,000)	(600,000)	(2,200,000)	(1,600,000)
Recoveries	3,000,000.0	1,800,000	4,100,000	4,000,000.0
Net (Charge-Offs) Recoveries	2,600,000	1,200,000	1,900,000	2,400,000
Provision for Credit Losses	60,400,000	(4,900,000)	106,000,000.0	(4,200,000)
Balance at End of Period	210,200,000	110,800,000	210,200,000	110,800,000
UNDRAWN LOAN COMMITMENTS AND STANDBY LETTERS OF CREDIT
Balance at Beginning of Period	40,400,000	24,900,000	19,900,000	25,600,000
Charge-Offs	0	0	0	0
Recoveries	0	0	0	0
Net (Charge-Offs) Recoveries	0	0	0	0
Provision for Credit Losses	8,600,000	(1,600,000)	20,200,000	(2,300,000)
Balance at End of Period	49,000,000.0	23,300,000	49,000,000.0	23,300,000
DEBT SECURITIES HELD TO MATURITY
Balance at Beginning of Period	9,500,000		0
Charge-Offs	0		0
Recoveries	0		0
Net (Charge-Offs) Recoveries	0		0
Provision for Credit Losses	(3,000,000.0)		(100,000)
Balance at End of Period	6,500,000		6,500,000
OTHER FINANCIAL ASSETS
Balance at Beginning of Period	1,100,000		0
Charge-Offs	0		0
Recoveries	0		0
Net (Charge-Offs) Recoveries	0		0
Provision for Credit Losses	200,000		900,000
Balance at End of Period	1,300,000		1,300,000
TOTAL
Balance at Beginning of Period	198,200,000	139,400,000	124,400,000	138,200,000
Charge-Offs	(400,000)	(600,000)	(2,200,000)	(1,600,000)
Recoveries	3,000,000.0	1,800,000	4,100,000	4,000,000.0
Net (Charge-Offs) Recoveries			1,900,000	2,400,000
Provision for Credit Losses	66,200,000	(6,500,000)	127,000,000.0	(6,500,000)
Balance at End of Period	267,000,000.0	$ 134,100,000	267,000,000.0	$ 134,100,000
Corporate Debt
DEBT SECURITIES HELD TO MATURITY
Balance at Beginning of Period	2,700,000		0
Provision for Credit Losses	(2,200,000)		(300,000)
Balance at End of Period	500,000		500,000
TOTAL
Provision for Credit Losses	$ (200,000)		0
Cumulative Effect Adjustment
LOANS AND LEASES
Balance at Beginning of Period			(2,200,000)
UNDRAWN LOAN COMMITMENTS AND STANDBY LETTERS OF CREDIT
Balance at Beginning of Period			8,900,000
DEBT SECURITIES HELD TO MATURITY
Balance at Beginning of Period			6,600,000
OTHER FINANCIAL ASSETS
Balance at Beginning of Period			400,000
TOTAL
Balance at Beginning of Period			13,700,000
Cumulative Effect Adjustment | Corporate Debt
DEBT SECURITIES HELD TO MATURITY
Balance at Beginning of Period			800,000
Adjusted Balance
LOANS AND LEASES
Balance at Beginning of Period			102,300,000
UNDRAWN LOAN COMMITMENTS AND STANDBY LETTERS OF CREDIT
Balance at Beginning of Period			28,800,000
DEBT SECURITIES HELD TO MATURITY
Balance at Beginning of Period			6,600,000
OTHER FINANCIAL ASSETS
Balance at Beginning of Period			400,000
TOTAL
Balance at Beginning of Period			138,100,000
Adjusted Balance | Corporate Debt
DEBT SECURITIES HELD TO MATURITY
Balance at Beginning of Period			$ 800,000