Investment Securities - Gross Unrealized Losses and Fair Values of Investment Securities (Details) - USD ($) $ in Millions	Sep. 30, 2019	Dec. 31, 2018
Available-for-Sale:
Available-for-Sale, Less than 12 Months, Fair Value	$ 588.8	$ 1,123.2
Available-for-sale, Less than 12 Months, Gross Unrealized Losses	(3.8)	(22.6)
Available-for-Sale, 12 Months or More, Fair Value	308.1	779.0
Available-for-Sale, 12 Months or More, Gross Unrealized Losses	(1.9)	(14.5)
Available-for-Sale, Total Fair Value	896.9	1,902.2
Available-for-Sale, Total Gross Unrealized Losses	(5.7)	(37.1)
Held-to-Maturity:
Held-to-Maturity, Less than 12 Months, Fair Value		90.4
Held-to-Maturity, Less Than 12 Months, Gross Unrealized Losses		(2.8)
Held-to-Maturity, 12 Months or More, Fair Value		216.9
Held-to-Maturity, 12 Months or More, Gross Unrealized Losses		(5.5)
Held-to-Maturity, Total Fair Value	0.0	307.3
Held-to-Maturity, Total Gross Unrealized Losses		(8.3)
State, county and municipal securities
Held-to-Maturity:
Held-to-Maturity, Less than 12 Months, Fair Value		25.9
Held-to-Maturity, Less Than 12 Months, Gross Unrealized Losses		(0.3)
Held-to-Maturity, 12 Months or More, Fair Value		57.1
Held-to-Maturity, 12 Months or More, Gross Unrealized Losses		(0.6)
Held-to-Maturity, Total Fair Value		83.0
Held-to-Maturity, Total Gross Unrealized Losses		(0.9)
Obligations of U.S. government agencies
Available-for-Sale:
Available-for-Sale, Less than 12 Months, Fair Value	136.1	363.1
Available-for-sale, Less than 12 Months, Gross Unrealized Losses	(0.5)	(7.9)
Available-for-Sale, 12 Months or More, Fair Value	85.7	154.5
Available-for-Sale, 12 Months or More, Gross Unrealized Losses	(0.3)	(2.3)
Available-for-Sale, Total Fair Value	221.8	517.6
Available-for-Sale, Total Gross Unrealized Losses	(0.8)	(10.2)
Held-to-Maturity:
Held-to-Maturity, Less than 12 Months, Fair Value		19.5
Held-to-Maturity, Less Than 12 Months, Gross Unrealized Losses		(0.3)
Held-to-Maturity, 12 Months or More, Fair Value		0.0
Held-to-Maturity, 12 Months or More, Gross Unrealized Losses		0.0
Held-to-Maturity, Total Fair Value		19.5
Held-to-Maturity, Total Gross Unrealized Losses		(0.3)
U.S. agency residential mortgage-backed securities & collateralized mortgage obligations
Available-for-Sale:
Available-for-Sale, Less than 12 Months, Fair Value	452.7	735.2
Available-for-sale, Less than 12 Months, Gross Unrealized Losses	(3.3)	(14.5)
Available-for-Sale, 12 Months or More, Fair Value	187.0	503.7
Available-for-Sale, 12 Months or More, Gross Unrealized Losses	(1.2)	(9.6)
Available-for-Sale, Total Fair Value	639.7	1,238.9
Available-for-Sale, Total Gross Unrealized Losses	(4.5)	(24.1)
Held-to-Maturity:
Held-to-Maturity, Less than 12 Months, Fair Value		45.0
Held-to-Maturity, Less Than 12 Months, Gross Unrealized Losses		(2.2)
Held-to-Maturity, 12 Months or More, Fair Value		120.2
Held-to-Maturity, 12 Months or More, Gross Unrealized Losses		(4.3)
Held-to-Maturity, Total Fair Value		165.2
Held-to-Maturity, Total Gross Unrealized Losses		(6.5)
Private mortgage-backed securities
Available-for-Sale:
Available-for-Sale, Less than 12 Months, Fair Value	0.0	0.0
Available-for-sale, Less than 12 Months, Gross Unrealized Losses	0.0	0.0
Available-for-Sale, 12 Months or More, Fair Value	35.4	69.4
Available-for-Sale, 12 Months or More, Gross Unrealized Losses	(0.4)	(1.8)
Available-for-Sale, Total Fair Value	35.4	69.4
Available-for-Sale, Total Gross Unrealized Losses	$ (0.4)	(1.8)
Corporate securities
Available-for-Sale:
Available-for-Sale, Less than 12 Months, Fair Value		24.9
Available-for-sale, Less than 12 Months, Gross Unrealized Losses		(0.2)
Available-for-Sale, 12 Months or More, Fair Value		51.4
Available-for-Sale, 12 Months or More, Gross Unrealized Losses		(0.8)
Available-for-Sale, Total Fair Value		76.3
Available-for-Sale, Total Gross Unrealized Losses		(1.0)
Held-to-Maturity:
Held-to-Maturity, Less than 12 Months, Fair Value		0.0
Held-to-Maturity, Less Than 12 Months, Gross Unrealized Losses		0.0
Held-to-Maturity, 12 Months or More, Fair Value		39.6
Held-to-Maturity, 12 Months or More, Gross Unrealized Losses		(0.6)
Held-to-Maturity, Total Fair Value		39.6
Held-to-Maturity, Total Gross Unrealized Losses		$ (0.6)

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Amount of accumulated unrealized loss on investments in debt and equity securities classified as available-for-sale that have been in a continuous loss position for less than twelve months.

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Amount of accumulated unrealized loss on investment in debt security measured at amortized cost (held-to-maturity), in continuous loss position for 12 months or longer.

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