Investment Securities - Amortized Cost and Approximate Fair Values of Investment Securities (Details) - USD ($) $ in Millions	Sep. 30, 2021	Dec. 31, 2020
Schedule of Available for Sale and Held-to-Maturity Securities [Line Items]
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	$ 0.0	$ 0.0
Available-for-Sale:
Amortized Cost	4,408.8	3,932.7
Gross Unrealized Gains	33.1	81.7
Gross Unrealized Losses	(38.3)	(5.7)
Estimated Fair Value	4,403.6	4,008.7
Held-to-Maturity:
Amortized Cost	1,618.1	51.6
Gross Unrealized Gains	23.3	3.4
Gross Unrealized Losses	(8.9)	0.0
Estimated Fair Value	1,632.5	55.0
U.S. Treasury notes
Available-for-Sale:
Amortized Cost	697.4
Gross Unrealized Gains	0.0
Gross Unrealized Losses	(5.6)
Estimated Fair Value	691.8
State, county and municipal securities
Available-for-Sale:
Amortized Cost	444.9	462.1
Gross Unrealized Gains	2.1	4.8
Gross Unrealized Losses	(7.1)	(1.0)
Estimated Fair Value	439.9	465.9
Held-to-Maturity:
Amortized Cost	69.7	46.6
Gross Unrealized Gains	2.3	3.2
Gross Unrealized Losses	(0.5)	0.0
Estimated Fair Value	71.5	49.8
Obligations of U.S. government agencies
Available-for-Sale:
Amortized Cost	332.3	332.9
Gross Unrealized Gains	0.2	1.0
Gross Unrealized Losses	(6.6)	(2.0)
Estimated Fair Value	325.9	331.9
U.S. agency residential mortgage-backed securities & collateralized mortgage obligations
Available-for-Sale:
Amortized Cost	2,021.4	2,830.8
Gross Unrealized Gains	25.9	69.3
Gross Unrealized Losses	(16.0)	(2.5)
Estimated Fair Value	2,031.3	2,897.6
Held-to-Maturity:
Amortized Cost	1,537.5	1.0
Gross Unrealized Gains	20.5	0.1
Gross Unrealized Losses	(8.4)	0.0
Estimated Fair Value	1,549.6	1.1
Private mortgage-backed securities
Available-for-Sale:
Amortized Cost	83.7	10.9
Gross Unrealized Gains	0.1	0.1
Gross Unrealized Losses	(0.3)	(0.1)
Estimated Fair Value	83.5	10.9
Corporate securities
Available-for-Sale:
Amortized Cost	376.7	295.8
Gross Unrealized Gains	4.2	6.5
Gross Unrealized Losses	(2.7)	(0.1)
Estimated Fair Value	378.2	302.2
Held-to-Maturity:
Amortized Cost	10.9	3.9
Gross Unrealized Gains	0.5	0.1
Gross Unrealized Losses	0.0	0.0
Estimated Fair Value	$ 11.4	4.0
Other investments
Available-for-Sale:
Amortized Cost		0.2
Gross Unrealized Gains		0.0
Gross Unrealized Losses		0.0
Estimated Fair Value		0.2
Other Investments
Held-to-Maturity:
Amortized Cost		0.1
Gross Unrealized Gains		0.0
Gross Unrealized Losses		0.0
Estimated Fair Value		$ 0.1