Investment Securities - Amortized Cost and Approximate Fair Values of Investment Securities (Details) - USD ($) $ in Millions	Sep. 30, 2019	Dec. 31, 2018
Available-for-Sale:
Amortized Cost	$ 2,714.7	$ 2,305.2
Gross Unrealized Gains	23.2	2.6
Gross Unrealized Losses	(5.7)	(37.1)
Estimated Fair Value	2,732.2	2,270.7
Held-to-Maturity:
Amortized Cost	94.9	406.8
Gross Unrealized Gains	2.3	2.2
Gross Unrealized Losses	0.0	(8.3)
Estimated Fair Value	97.2	400.7
U.S. Treasury Notes
Available-for-Sale:
Amortized Cost	7.0	2.6
Gross Unrealized Gains	0.0	0.0
Gross Unrealized Losses	0.0	0.0
Estimated Fair Value	7.0	2.6
State, county and municipal securities
Available-for-Sale:
Amortized Cost	77.4
Gross Unrealized Gains	1.0
Gross Unrealized Losses	0.0
Estimated Fair Value	78.4
Held-to-Maturity:
Amortized Cost	59.7	150.9
Gross Unrealized Gains	2.3	1.8
Gross Unrealized Losses	0.0	(0.9)
Estimated Fair Value	62.0	151.8
Obligations of U.S. government agencies
Available-for-Sale:
Amortized Cost	427.2	569.3
Gross Unrealized Gains	0.3	0.1
Gross Unrealized Losses	(0.8)	(10.2)
Estimated Fair Value	426.7	559.2
Held-to-Maturity:
Amortized Cost	19.9	19.8
Gross Unrealized Gains	0.0	0.0
Gross Unrealized Losses	0.0	(0.3)
Estimated Fair Value	19.9	19.5
U.S. agency residential mortgage-backed securities & collateralized mortgage obligations
Available-for-Sale:
Amortized Cost	2,046.6	1,566.4
Gross Unrealized Gains	21.1	2.5
Gross Unrealized Losses	(4.5)	(24.1)
Estimated Fair Value	2,063.2	1,544.8
Held-to-Maturity:
Amortized Cost	1.2	189.7
Gross Unrealized Gains	0.0	0.3
Gross Unrealized Losses	0.0	(6.5)
Estimated Fair Value	1.2	183.5
Private mortgage-backed securities
Available-for-Sale:
Amortized Cost	54.4	72.0
Gross Unrealized Gains	0.1	0.0
Gross Unrealized Losses	(0.4)	(1.8)
Estimated Fair Value	54.1	70.2
Corporate securities
Available-for-Sale:
Amortized Cost	96.0	92.9
Gross Unrealized Gains	0.7	0.0
Gross Unrealized Losses	0.0	(1.0)
Estimated Fair Value	96.7	91.9
Held-to-Maturity:
Amortized Cost	14.0	46.3
Gross Unrealized Gains	0.0	0.1
Gross Unrealized Losses	0.0	(0.6)
Estimated Fair Value	14.0	45.8
Other investments
Available-for-Sale:
Amortized Cost	6.1	2.0
Gross Unrealized Gains	0.0	0.0
Gross Unrealized Losses	0.0	0.0
Estimated Fair Value	6.1	2.0
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

Amount, before tax, of unrealized gain in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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