Investment Securities - Amortized Cost and Approximate Fair Values of Investment Securities (Details) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2020	Dec. 31, 2019
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
Gain (Loss) on Disposition of Assets	0.0
Available-for-Sale:
Amortized Cost	3,369.2	$ 2,945.5
Gross Unrealized Gains	86.2	21.7
Gross Unrealized Losses	(1.9)	(7.2)
Estimated Fair Value	3,453.5	2,960.0
Held-to-Maturity:
Amortized Cost	55.0	92.3
Gross Unrealized Gains	3.4	2.2
Gross Unrealized Losses	0.0	0.0
Estimated Fair Value	58.4	94.5
U.S. Treasury Notes
Available-for-Sale:
Amortized Cost	8.0	9.0
Gross Unrealized Gains	0.0	0.0
Gross Unrealized Losses	0.0	0.0
Estimated Fair Value	8.0	9.0
State, county and municipal securities
Available-for-Sale:
Amortized Cost	233.6	80.1
Gross Unrealized Gains	2.7	0.8
Gross Unrealized Losses	(0.2)	0.0
Estimated Fair Value	236.1	80.9
Held-to-Maturity:
Amortized Cost	49.9	57.3
Gross Unrealized Gains	3.2	2.1
Gross Unrealized Losses	0.0	0.0
Estimated Fair Value	53.1	59.4
Obligations of U.S. government agencies
Available-for-Sale:
Amortized Cost	278.4	367.5
Gross Unrealized Gains	1.0	0.1
Gross Unrealized Losses	(0.6)	(0.8)
Estimated Fair Value	278.8	366.8
Held-to-Maturity:
Amortized Cost		19.8
Gross Unrealized Gains		0.0
Gross Unrealized Losses		0.0
Estimated Fair Value		19.8
U.S. agency residential mortgage-backed securities & collateralized mortgage obligations
Available-for-Sale:
Amortized Cost	2,535.3	2,303.6
Gross Unrealized Gains	76.9	19.6
Gross Unrealized Losses	(0.5)	(6.0)
Estimated Fair Value	2,611.7	2,317.2
Held-to-Maturity:
Amortized Cost	1.0	1.2
Gross Unrealized Gains	0.1	0.0
Gross Unrealized Losses	0.0	0.0
Estimated Fair Value	1.1	1.2
Private mortgage-backed securities
Available-for-Sale:
Amortized Cost	20.0	47.6
Gross Unrealized Gains	0.2	0.0
Gross Unrealized Losses	(0.1)	(0.4)
Estimated Fair Value	20.1	47.2
Corporate securities
Available-for-Sale:
Amortized Cost	292.9	134.5
Gross Unrealized Gains	5.4	1.2
Gross Unrealized Losses	(0.5)	0.0
Estimated Fair Value	297.8	135.7
Held-to-Maturity:
Amortized Cost	4.0	13.9
Gross Unrealized Gains	0.1	0.1
Gross Unrealized Losses	0.0	0.0
Estimated Fair Value	4.1	14.0
Other investments
Available-for-Sale:
Amortized Cost	1.0	3.2
Gross Unrealized Gains	0.0	0.0
Gross Unrealized Losses	0.0	0.0
Estimated Fair Value	1.0	3.2
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