Investment Securities - Amortized Cost and Approximate Fair Values of Investment Securities (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
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	$ 48.8
Available-for-Sale:
Amortized Cost	7,573.5	$ 4,859.3
Gross Unrealized Gains	2.3	20.6
Gross Unrealized Losses	(629.7)	(59.4)
Estimated Fair Value	6,946.1	4,820.5
Held-to-Maturity:
Amortized Cost	3,453.7	1,687.6
Allowance for Credit Losses	0.4	15.6
Gross Unrealized Losses	(400.0)	(35.7)
Held-to-maturity investment securities, estimated fair value	3,052.2	1,667.5
Debt Securities, Held-to-Maturity, Transfer, Unamortized Gain	14.2	20.1
Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, Rolling within One Year, Fair Value	48.1
Debt Securities, Held-to-maturity, Maturity, Allocated and Single Maturity Date, within One Year, Amortized Cost	2.5
Debt Securities, Held-to-maturity, Maturity, Allocated and Single Maturity Date, Rolling within One Year, Fair Value	2.5
Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, Rolling after One Through Five Years, Amortized Cost	1,478.9
Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, Rolling after One Through Five Years, Fair Value	1,419.8
Debt Securities, Held-to-maturity, Maturity, Allocated and Single Maturity Date, after One Through Five Years, Amortized Cost	607.6
Debt Securities, Held-to-maturity, Maturity, Allocated and Single Maturity Date, Rolling after One Through Five Years, Fair Value	581.6
Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, Rolling after Five Through Ten Years, Amortized Cost	1,750.0
Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, Rolling after Five Through Ten Years, Fair Value	1,564.3
Debt Securities, Held-to-maturity, Maturity, Allocated and Single Maturity Date, after Five Through Ten Years, Amortized Cost	732.7
Debt Securities, Held-to-maturity, Maturity, Allocated and Single Maturity Date, Rolling after Five Through Ten Years, Fair Value	639.5
Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, Rolling after 10 Years, Amortized Cost	4,295.8
Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, Rolling after 10 Years, Fair Value	3,913.9
Debt Securities, Held-to-maturity, Maturity, Allocated and Single Maturity Date, after 10 Years, Amortized Cost	2,110.9
Debt Securities, Held-to-maturity, Maturity, Allocated and Single Maturity Date, Rolling after 10 Years, Fair Value	1,828.6
Debt Securities, Held-to-maturity, Allowance for Credit Loss	(1.9)	0.0	$ 0.0	$ 0.0
Debt Securities, Held-to-Maturity, Amortized Cost, after Allowance for Credit Loss	3,451.8	1,687.6
Debt and Equity Securities, Unrealized Gain (Loss)	(20.2)
US Treasury notes
Available-for-Sale:
Amortized Cost	675.1	697.6
Gross Unrealized Gains	2.1	0.0
Gross Unrealized Losses	(34.5)	(12.9)
Estimated Fair Value	642.7	684.7
Held-to-Maturity:
Amortized Cost	396.6
Allowance for Credit Losses	0.0
Gross Unrealized Losses	(10.2)
Held-to-maturity investment securities, estimated fair value	386.4
Debt Securities, Held-to-maturity, Allowance for Credit Loss	0.0
Debt Securities, Held-to-Maturity, Amortized Cost, after Allowance for Credit Loss	396.6
State, county and municipal securities
Available-for-Sale:
Amortized Cost	314.3	434.7
Gross Unrealized Gains	0.0	2.1
Gross Unrealized Losses	(50.6)	(9.3)
Estimated Fair Value	263.7	427.5
Held-to-Maturity:
Amortized Cost	181.2	67.6
Allowance for Credit Losses	0.2	2.0
Gross Unrealized Losses	(31.6)	(0.4)
Held-to-maturity investment securities, estimated fair value	149.7	69.2
Debt Securities, Held-to-maturity, Allowance for Credit Loss	(0.1)	0.0
Debt Securities, Held-to-Maturity, Amortized Cost, after Allowance for Credit Loss	181.1	67.6
US Government Agencies Debt Securities [Member]
Available-for-Sale:
Amortized Cost	216.2	356.0
Gross Unrealized Gains	0.0	0.1
Gross Unrealized Losses	(17.3)	(9.2)
Estimated Fair Value	198.9	346.9
Held-to-Maturity:
Amortized Cost	351.7
Allowance for Credit Losses	0.0
Gross Unrealized Losses	(49.6)
Held-to-maturity investment securities, estimated fair value	302.1
Debt Securities, Held-to-maturity, Allowance for Credit Loss	0.0
Debt Securities, Held-to-Maturity, Amortized Cost, after Allowance for Credit Loss	351.7
Corporate Debt Securities [Member]
Available-for-Sale:
Amortized Cost	272.3	271.1
Gross Unrealized Gains	0.0	3.0
Gross Unrealized Losses	(30.8)	(3.6)
Estimated Fair Value	241.5	270.5
Held-to-Maturity:
Amortized Cost	80.1	11.0
Allowance for Credit Losses	0.0	0.4
Gross Unrealized Losses	(7.0)	0.0
Held-to-maturity investment securities, estimated fair value	71.3	11.4
Debt Securities, Held-to-maturity, Allowance for Credit Loss	(1.8)	0.0
Debt Securities, Held-to-Maturity, Amortized Cost, after Allowance for Credit Loss	78.3	11.0
Mortgage-backed Securities, Issued by US Government Sponsored Enterprises [Member]
Available-for-Sale:
Amortized Cost	4,685.5	2,027.3
Gross Unrealized Gains	0.2	14.1
Gross Unrealized Losses	(426.0)	(23.3)
Estimated Fair Value	4,259.7	2,018.1
Held-to-Maturity:
Amortized Cost	2,444.1	1,609.0
Allowance for Credit Losses	0.2	13.2
Gross Unrealized Losses	(301.6)	(35.3)
Held-to-maturity investment securities, estimated fair value	2,142.7	1,586.9
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	2,444.1	1,609.0
Mortgage-backed Securities, Issued by Private Enterprises [Member]
Available-for-Sale:
Amortized Cost	264.9	174.4
Gross Unrealized Gains	0.0	0.1
Gross Unrealized Losses	(36.9)	(1.1)
Estimated Fair Value	228.0	173.4
Collateralized Loan Obligations
Available-for-Sale:
Amortized Cost	1,145.2	898.2
Gross Unrealized Gains	0.0	1.2
Gross Unrealized Losses	(33.6)	0.0
Estimated Fair Value	1,111.6	$ 899.4
Callable Within One Year [Member]
Schedule of Available for Sale and Held-to-Maturity Securities
Investment Securities, Primarily Classified As Available-For-Sale, Debt Maturities, Rolling Year Two Through Five, Amortized Cost Basis	1,443.3
Investment Securities, Primarily Classified As Available-For-Sale, Debt Maturities, Rolling Year Two Through Five, Fair Value	1,356.5
Held-to-Maturity:
Investment Securities, Callable Structured, Amortized Cost Basis	$ 0.0