Investment Securities - Amortized Cost and Approximate Fair Values of Investment Securities (Details) - USD ($) $ in Millions	Mar. 31, 2020	Dec. 31, 2019
Available-for-Sale:
Amortized Cost	$ 2,946.7	$ 2,945.5
Gross Unrealized Gains	63.1	21.7
Gross Unrealized Losses	(9.5)	(7.2)
Estimated Fair Value	3,000.3	2,960.0
Held-to-Maturity:
Amortized Cost	70.2	92.3
Gross Unrealized Gains	2.4	2.2
Gross Unrealized Losses	0.0	0.0
Estimated Fair Value	72.6	94.5
U.S. Treasury Notes
Available-for-Sale:
Amortized Cost	17.1	9.0
Gross Unrealized Gains	0.0	0.0
Gross Unrealized Losses	0.0	0.0
Estimated Fair Value	17.1	9.0
State, county and municipal securities
Available-for-Sale:
Amortized Cost	111.4	80.1
Gross Unrealized Gains	0.9	0.8
Gross Unrealized Losses	0.0	0.0
Estimated Fair Value	112.3	80.9
Held-to-Maturity:
Amortized Cost	55.2	57.3
Gross Unrealized Gains	2.3	2.1
Gross Unrealized Losses	0.0	0.0
Estimated Fair Value	57.5	59.4
Obligations of U.S. government agencies
Available-for-Sale:
Amortized Cost	202.7	367.5
Gross Unrealized Gains	1.0	0.1
Gross Unrealized Losses	(0.1)	(0.8)
Estimated Fair Value	203.6	366.8
Held-to-Maturity:
Amortized Cost	9.8	19.8
Gross Unrealized Gains	0.0	0.0
Gross Unrealized Losses	0.0	0.0
Estimated Fair Value	9.8	19.8
U.S. agency residential mortgage-backed securities & collateralized mortgage obligations
Available-for-Sale:
Amortized Cost	2,356.7	2,303.6
Gross Unrealized Gains	60.8	19.6
Gross Unrealized Losses	(2.7)	(6.0)
Estimated Fair Value	2,414.8	2,317.2
Held-to-Maturity:
Amortized Cost	1.2	1.2
Gross Unrealized Gains	0.1	0.0
Gross Unrealized Losses	0.0	0.0
Estimated Fair Value	1.3	1.2
Private mortgage-backed securities
Available-for-Sale:
Amortized Cost	41.2	47.6
Gross Unrealized Gains	0.0	0.0
Gross Unrealized Losses	(1.2)	(0.4)
Estimated Fair Value	40.0	47.2
Corporate securities
Available-for-Sale:
Amortized Cost	215.6	134.5
Gross Unrealized Gains	0.4	1.2
Gross Unrealized Losses	(5.5)	0.0
Estimated Fair Value	210.5	135.7
Held-to-Maturity:
Amortized Cost	3.9	13.9
Gross Unrealized Gains	0.0	0.1
Gross Unrealized Losses	0.0	0.0
Estimated Fair Value	3.9	14.0
Other investments
Available-for-Sale:
Amortized Cost	2.0	3.2
Gross Unrealized Gains	0.0	0.0
Gross Unrealized Losses	0.0	0.0
Estimated Fair Value	2.0	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

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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Amount, before tax, of unrealized loss 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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