Investment Securities - Amortized Cost and Approximate Fair Values of Investment Securities (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020
Schedule of Available for Sale and Held-to-Maturity Securities
Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, Rolling within One Year, Amortized Cost	$ 40.1
Investment Securities, Primarily Classified As Available-For-Sale, Debt Maturities, Rolling Year Two Through Five, Amortized Cost Basis	1,603.3
Investment Securities, Primarily Classified As Available-For-Sale, Debt Maturities, Rolling Year Two Through Five, Fair Value	1,552.2
Available-for-Sale:
Gross Unrealized Gains	3.2	$ 2.3
Gross Unrealized Losses	(469.5)	(629.7)
Held-to-Maturity:
Debt Securities, Held-to-maturity	3,208.7	3,453.7
Allowance for Credit Losses	0.4	0.4
Gross Unrealized Losses	(334.3)	(400.0)
Held-to-maturity investment securities, estimated fair value	2,874.0	3,052.2
Debt Securities, Held-to-Maturity, Transfer, Unamortized Gain	10.7
Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, Rolling within One Year, Fair Value	39.4
Debt Securities, Held-to-maturity, Maturity, Allocated and Single Maturity Date, within One Year, Amortized Cost	309.1
Debt Securities, Held-to-maturity, Maturity, Allocated and Single Maturity Date, Rolling within One Year, Fair Value	307.5
Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, Rolling after One Through Five Years, Amortized Cost	1,004.4
Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, Rolling after One Through Five Years, Fair Value	938.7
Debt Securities, Held-to-maturity, Maturity, Allocated and Single Maturity Date, after One Through Five Years, Amortized Cost	346.6
Debt Securities, Held-to-maturity, Maturity, Allocated and Single Maturity Date, Rolling after One Through Five Years, Fair Value	329.4
Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, Rolling after Five Through Ten Years, Amortized Cost	1,373.9
Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, Rolling after Five Through Ten Years, Fair Value	1,255.4
Debt Securities, Held-to-maturity, Maturity, Allocated and Single Maturity Date, after Five Through Ten Years, Amortized Cost	654.0
Debt Securities, Held-to-maturity, Maturity, Allocated and Single Maturity Date, Rolling after Five Through Ten Years, Fair Value	579.1
Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, Rolling after 10 Years, Amortized Cost	3,889.4
Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, Rolling after 10 Years, Fair Value	3,608.0
Debt Securities, Held-to-maturity, Maturity, Allocated and Single Maturity Date, after 10 Years, Amortized Cost	1,899.0
Debt Securities, Held-to-maturity, Maturity, Allocated and Single Maturity Date, Rolling after 10 Years, Fair Value	1,658.0
Debt Securities, Held-to-maturity, Allowance for Credit Loss	(0.8)	(1.9)	$ 0.0	$ 0.0
Debt Securities, Held-to-Maturity, Amortized Cost, after Allowance for Credit Loss	3,207.9	3,451.8
Debt and Equity Securities, Unrealized Gain (Loss)	(18.1)	(20.2)
Debt Securities, Available-for-Sale, Amortized Cost, Excluding Accrued Interest, before Allowance for Credit Loss	6,307.8	7,573.5
Debt Securities, Available-for-Sale, Allowance for Credit Loss, Excluding Accrued Interest	0.0	0.0
Debt Securities, Available-for-Sale, Amortized Cost, Excluding Accrued Interest, after Allowance for Credit Loss	6,307.8	7,573.5
Debt Securities, Available-for-Sale, Excluding Accrued Interest	5,841.5	6,946.1
US Treasury notes
Available-for-Sale:
Gross Unrealized Gains	0.0	2.1
Gross Unrealized Losses	(25.5)	(34.5)
Held-to-Maturity:
Debt Securities, Held-to-maturity	399.0	396.6
Allowance for Credit Losses	0.0	0.0
Gross Unrealized Losses	(2.8)	(10.2)
Held-to-maturity investment securities, estimated fair value	396.2	386.4
Debt Securities, Held-to-maturity, Allowance for Credit Loss	0.0	0.0
Debt Securities, Held-to-Maturity, Amortized Cost, after Allowance for Credit Loss	399.0	396.6
Debt Securities, Available-for-Sale, Amortized Cost, Excluding Accrued Interest, before Allowance for Credit Loss	250.2	675.1
Debt Securities, Available-for-Sale, Allowance for Credit Loss, Excluding Accrued Interest	0.0	0.0
Debt Securities, Available-for-Sale, Amortized Cost, Excluding Accrued Interest, after Allowance for Credit Loss	250.2	675.1
Debt Securities, Available-for-Sale, Excluding Accrued Interest	224.7	642.7
State, county and municipal securities
Available-for-Sale:
Gross Unrealized Gains	0.0	0.0
Gross Unrealized Losses	(36.9)	(50.6)
Held-to-Maturity:
Debt Securities, Held-to-maturity	179.2	181.2
Allowance for Credit Losses	0.2	0.2
Gross Unrealized Losses	(24.2)	(31.6)
Held-to-maturity investment securities, estimated fair value	155.2	149.7
Debt Securities, Held-to-maturity, Allowance for Credit Loss	0.0	(0.1)
Debt Securities, Held-to-Maturity, Amortized Cost, after Allowance for Credit Loss	179.2	181.1
Debt Securities, Available-for-Sale, Amortized Cost, Excluding Accrued Interest, before Allowance for Credit Loss	256.7	314.3
Debt Securities, Available-for-Sale, Allowance for Credit Loss, Excluding Accrued Interest	0.0	0.0
Debt Securities, Available-for-Sale, Amortized Cost, Excluding Accrued Interest, after Allowance for Credit Loss	256.7	314.3
Debt Securities, Available-for-Sale, Excluding Accrued Interest	219.8	263.7
US Government Agencies Debt Securities [Member]
Available-for-Sale:
Gross Unrealized Gains	0.0	0.0
Gross Unrealized Losses	(10.9)	(17.3)
Held-to-Maturity:
Debt Securities, Held-to-maturity	354.5	351.7
Allowance for Credit Losses	0.0	0.0
Gross Unrealized Losses	(42.4)	(49.6)
Held-to-maturity investment securities, estimated fair value	312.1	302.1
Debt Securities, Held-to-maturity, Allowance for Credit Loss	0.0	0.0
Debt Securities, Held-to-Maturity, Amortized Cost, after Allowance for Credit Loss	354.5	351.7
Debt Securities, Available-for-Sale, Amortized Cost, Excluding Accrued Interest, before Allowance for Credit Loss	179.4	216.2
Debt Securities, Available-for-Sale, Allowance for Credit Loss, Excluding Accrued Interest	0.0	0.0
Debt Securities, Available-for-Sale, Amortized Cost, Excluding Accrued Interest, after Allowance for Credit Loss	179.4	216.2
Debt Securities, Available-for-Sale, Excluding Accrued Interest	168.5	198.9
Corporate Debt Securities [Member]
Available-for-Sale:
Gross Unrealized Gains	0.0	0.0
Gross Unrealized Losses	(24.4)	(30.8)
Held-to-Maturity:
Debt Securities, Held-to-maturity	57.0	80.1
Allowance for Credit Losses	0.0	0.0
Gross Unrealized Losses	(6.0)	(7.0)
Held-to-maturity investment securities, estimated fair value	50.2	71.3
Debt Securities, Held-to-maturity, Allowance for Credit Loss	(0.8)	(1.8)
Debt Securities, Held-to-Maturity, Amortized Cost, after Allowance for Credit Loss	56.2	78.3
Debt Securities, Available-for-Sale, Amortized Cost, Excluding Accrued Interest, before Allowance for Credit Loss	260.8	272.3
Debt Securities, Available-for-Sale, Allowance for Credit Loss, Excluding Accrued Interest	0.0	0.0
Debt Securities, Available-for-Sale, Amortized Cost, Excluding Accrued Interest, after Allowance for Credit Loss	260.8	272.3
Debt Securities, Available-for-Sale, Excluding Accrued Interest	236.4	241.5
Mortgage-backed Securities, Issued by Private Enterprises [Member]
Available-for-Sale:
Gross Unrealized Gains	0.0	0.0
Gross Unrealized Losses	(30.9)	(36.9)
Held-to-Maturity:
Debt Securities, Available-for-Sale, Amortized Cost, Excluding Accrued Interest, before Allowance for Credit Loss	241.3	264.9
Debt Securities, Available-for-Sale, Allowance for Credit Loss, Excluding Accrued Interest	0.0	0.0
Debt Securities, Available-for-Sale, Amortized Cost, Excluding Accrued Interest, after Allowance for Credit Loss	241.3	264.9
Debt Securities, Available-for-Sale, Excluding Accrued Interest	210.4	228.0
Collateralized Loan Obligations
Available-for-Sale:
Gross Unrealized Gains	0.1	0.0
Gross Unrealized Losses	(2.3)	(33.6)
Held-to-Maturity:
Debt Securities, Available-for-Sale, Amortized Cost, Excluding Accrued Interest, before Allowance for Credit Loss	1,121.9	1,145.2
Debt Securities, Available-for-Sale, Allowance for Credit Loss, Excluding Accrued Interest	0.0	0.0
Debt Securities, Available-for-Sale, Amortized Cost, Excluding Accrued Interest, after Allowance for Credit Loss	1,121.9	1,145.2
Debt Securities, Available-for-Sale, Excluding Accrued Interest	1,119.7	1,111.6
Residential Mortgage-Backed Securities
Available-for-Sale:
Gross Unrealized Gains	1.2	0.0
Gross Unrealized Losses	(130.6)	(167.7)
Held-to-Maturity:
Debt Securities, Held-to-maturity	1,232.6	1,391.5
Allowance for Credit Losses	0.0	0.0
Gross Unrealized Losses	(137.0)	(166.6)
Held-to-maturity investment securities, estimated fair value	1,095.6	1,224.9
Debt Securities, Held-to-maturity, Allowance for Credit Loss	0.0	0.0
Debt Securities, Held-to-Maturity, Amortized Cost, after Allowance for Credit Loss	1,232.6	1,391.5
Debt Securities, Available-for-Sale, Amortized Cost, Excluding Accrued Interest, before Allowance for Credit Loss	1,496.3	1,726.0
Debt Securities, Available-for-Sale, Allowance for Credit Loss, Excluding Accrued Interest	0.0	0.0
Debt Securities, Available-for-Sale, Amortized Cost, Excluding Accrued Interest, after Allowance for Credit Loss	1,496.3	1,726.0
Debt Securities, Available-for-Sale, Excluding Accrued Interest	1,366.9	1,558.3
Collateralized Mortgage Obligations
Available-for-Sale:
Gross Unrealized Gains	1.3	0.0
Gross Unrealized Losses	(120.3)	(141.3)
Held-to-Maturity:
Debt Securities, Held-to-maturity	475.9	525.8
Allowance for Credit Losses	0.2	0.2
Gross Unrealized Losses	(69.0)	(76.7)
Held-to-maturity investment securities, estimated fair value	407.1	449.3
Debt Securities, Held-to-maturity, Allowance for Credit Loss	0.0	0.0
Debt Securities, Held-to-Maturity, Amortized Cost, after Allowance for Credit Loss	475.9	525.8
Debt Securities, Available-for-Sale, Amortized Cost, Excluding Accrued Interest, before Allowance for Credit Loss	1,308.5	1,491.5
Debt Securities, Available-for-Sale, Allowance for Credit Loss, Excluding Accrued Interest	0.0	0.0
Debt Securities, Available-for-Sale, Amortized Cost, Excluding Accrued Interest, after Allowance for Credit Loss	1,308.5	1,491.5
Debt Securities, Available-for-Sale, Excluding Accrued Interest	1,189.5	1,350.2
Commercial Mortgage Backed Securities
Available-for-Sale:
Gross Unrealized Gains	0.6	0.2
Gross Unrealized Losses	(87.7)	(117.0)
Held-to-Maturity:
Debt Securities, Held-to-maturity	510.5	526.8
Allowance for Credit Losses	0.0	0.0
Gross Unrealized Losses	(52.9)	(58.3)
Held-to-maturity investment securities, estimated fair value	457.6	468.5
Debt Securities, Held-to-maturity, Allowance for Credit Loss	0.0	0.0
Debt Securities, Held-to-Maturity, Amortized Cost, after Allowance for Credit Loss	510.5	526.8
Debt Securities, Available-for-Sale, Amortized Cost, Excluding Accrued Interest, before Allowance for Credit Loss	1,192.7	1,468.0
Debt Securities, Available-for-Sale, Allowance for Credit Loss, Excluding Accrued Interest	0.0	0.0
Debt Securities, Available-for-Sale, Amortized Cost, Excluding Accrued Interest, after Allowance for Credit Loss	1,192.7	1,468.0
Debt Securities, Available-for-Sale, Excluding Accrued Interest	$ 1,105.6	1,351.2
US agency, residential and commercial MBS and CMOs
Held-to-Maturity:
Debt Securities, Held-to-Maturity, Transfer, Unamortized Gain		$ 14.2