Investment Securities - Amortized Cost and Approximate Fair Values of Investment Securities (Details) - USD ($) $ in Millions	Mar. 31, 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
Available-for-Sale:
Amortized Cost	4,011.0	$ 3,932.7
Gross Unrealized Gains	62.3	81.7
Gross Unrealized Losses	(53.2)	(5.7)
Estimated Fair Value	4,020.1	4,008.7
Held-to-Maturity:
Amortized Cost	866.3	51.6
Gross Unrealized Gains	3.0	3.4
Gross Unrealized Losses	(13.7)	0.0
Estimated Fair Value	855.6	55.0
U.S. Treasury Notes
Available-for-Sale:
Amortized Cost	1.0
Gross Unrealized Gains	0.0
Gross Unrealized Losses	0.0
Estimated Fair Value	1.0
State, county and municipal securities
Available-for-Sale:
Amortized Cost	458.0	462.1
Gross Unrealized Gains	1.8	4.8
Gross Unrealized Losses	(14.7)	(1.0)
Estimated Fair Value	445.1	465.9
Held-to-Maturity:
Amortized Cost	74.1	46.6
Gross Unrealized Gains	2.7	3.2
Gross Unrealized Losses	(0.9)	0.0
Estimated Fair Value	75.9	49.8
Obligations of U.S. government agencies
Available-for-Sale:
Amortized Cost	334.9	332.9
Gross Unrealized Gains	0.4	1.0
Gross Unrealized Losses	(11.8)	(2.0)
Estimated Fair Value	323.5	331.9
U.S. agency residential mortgage-backed securities & collateralized mortgage obligations
Available-for-Sale:
Amortized Cost	2,866.0	2,830.8
Gross Unrealized Gains	56.3	69.3
Gross Unrealized Losses	(20.8)	(2.5)
Estimated Fair Value	2,901.5	2,897.6
Held-to-Maturity:
Amortized Cost	788.2	1.0
Gross Unrealized Gains	0.2	0.1
Gross Unrealized Losses	(12.8)	0.0
Estimated Fair Value	775.6	1.1
Private mortgage-backed securities
Available-for-Sale:
Amortized Cost	6.5	10.9
Gross Unrealized Gains	0.1	0.1
Gross Unrealized Losses	0.0	(0.1)
Estimated Fair Value	6.6	10.9
Corporate securities
Available-for-Sale:
Amortized Cost	344.6	295.8
Gross Unrealized Gains	3.7	6.5
Gross Unrealized Losses	(5.9)	(0.1)
Estimated Fair Value	342.4	302.2
Held-to-Maturity:
Amortized Cost	3.9	3.9
Gross Unrealized Gains	0.1	0.1
Gross Unrealized Losses	0.0	0.0
Estimated Fair Value	4.0	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	0.1
Gross Unrealized Gains	0.0	0.0
Gross Unrealized Losses	0.0	0.0
Estimated Fair Value	$ 0.1	$ 0.1