Investment Securities - Schedule of Available-for-Sale Securities (Details) - USD ($) $ in Millions	Mar. 31, 2026	Dec. 31, 2025
Debt Securities, Available-for-sale [Line Items]
Amortized cost, current	$ 4,818.2	$ 4,328.8
Accumulated gross unrealized gain, current	29.6	54.3
Accumulated gross unrealized loss, current	72.4	50.2
Debt securities, current, fair value	4,775.4	4,332.9
Total	92.8	94.2
Executive deferred compensation plan trust	16.7	18.4
Asset Pledged as Collateral
Debt Securities, Available-for-sale [Line Items]
Fair value of debt securities pledged	86.5
Asset Pledged as Collateral \| FHLB advances \| Federal Reserve Bank Advances
Debt Securities, Available-for-sale [Line Items]
Fair value of debt securities pledged	1,800.0
Available-for-sale debt securities
Debt Securities, Available-for-sale [Line Items]
Amortized cost, non-current	53.6	54.8
Accumulated gross unrealized gain, non-current	0.1	0.2
Accumulated gross unrealized loss, non-current	1.6	1.3
Debt securities, noncurrent, fair value	52.2	53.7
U.S. treasury notes
Debt Securities, Available-for-sale [Line Items]
Amortized cost, current	417.8	412.4
Accumulated gross unrealized gain, current	0.4	1.0
Accumulated gross unrealized loss, current	16.8	15.9
Debt securities, current, fair value	401.4	397.5
Corporate and sovereign debt securities
Debt Securities, Available-for-sale [Line Items]
Amortized cost, current	1,596.2	1,322.8
Accumulated gross unrealized gain, current	15.4	29.7
Accumulated gross unrealized loss, current	15.2	2.9
Debt securities, current, fair value	1,596.3	1,349.6
Municipal bonds
Debt Securities, Available-for-sale [Line Items]
Amortized cost, current	70.6	68.7
Accumulated gross unrealized gain, current	0.7	0.8
Accumulated gross unrealized loss, current	3.1	2.9
Debt securities, current, fair value	68.2	66.6
Municipal bonds \| Estimate of Fair Value Measurement
Debt Securities, Available-for-sale [Line Items]
Fair value of debt securities pledged	44.9	45.3
Asset-backed securities
Debt Securities, Available-for-sale [Line Items]
Amortized cost, current	1,018.8	865.3
Accumulated gross unrealized gain, current	2.8	5.6
Accumulated gross unrealized loss, current	4.4	2.7
Debt securities, current, fair value	1,017.1	868.2
Mortgage-backed securities
Debt Securities, Available-for-sale [Line Items]
Amortized cost, current	1,714.9	1,659.6
Accumulated gross unrealized gain, current	10.4	17.1
Accumulated gross unrealized loss, current	32.9	25.7
Debt securities, current, fair value	1,692.4	1,651.0
Mortgage-backed securities \| Estimate of Fair Value Measurement
Debt Securities, Available-for-sale [Line Items]
Fair value of debt securities pledged	7.3	8.4
Mutual fund
Debt Securities, Available-for-sale [Line Items]
Fixed-income mutual fund and Pooled investment fund measured at NAV	27.7	27.6
Pooled investment fund
Debt Securities, Available-for-sale [Line Items]
Fixed-income mutual fund and Pooled investment fund measured at NAV	$ 12.9	$ 12.9

Amortized cost excluding accrued interest, after allowance for credit loss, of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), classified as current.

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