Investment Securities - Amortized Cost and Approximate Fair Values of Investment Securities (Details) - USD ($) $ in Millions	6 Months Ended
	Jun. 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	3,229.0	$ 2,945.5
Gross Unrealized Gains	90.3	21.7
Gross Unrealized Losses	(1.7)	(7.2)
Estimated Fair Value	3,317.6	2,960.0
Held-to-Maturity:
Amortized Cost	67.9	92.3
Gross Unrealized Gains	3.2	2.2
Gross Unrealized Losses	0.0	0.0
Estimated Fair Value	71.1	94.5
U.S. Treasury Notes
Available-for-Sale:
Amortized Cost	10.1	9.0
Gross Unrealized Gains	0.0	0.0
Gross Unrealized Losses	0.0	0.0
Estimated Fair Value	10.1	9.0
State, county and municipal securities
Available-for-Sale:
Amortized Cost	122.4	80.1
Gross Unrealized Gains	1.9	0.8
Gross Unrealized Losses	(0.1)	0.0
Estimated Fair Value	124.2	80.9
Held-to-Maturity:
Amortized Cost	52.9	57.3
Gross Unrealized Gains	2.9	2.1
Gross Unrealized Losses	0.0	0.0
Estimated Fair Value	55.8	59.4
Obligations of U.S. government agencies
Available-for-Sale:
Amortized Cost	264.4	367.5
Gross Unrealized Gains	1.1	0.1
Gross Unrealized Losses	0.0	(0.8)
Estimated Fair Value	265.5	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	2,528.0	2,303.6
Gross Unrealized Gains	82.3	19.6
Gross Unrealized Losses	(0.7)	(6.0)
Estimated Fair Value	2,609.6	2,317.2
Held-to-Maturity:
Amortized Cost	1.1	1.2
Gross Unrealized Gains	0.1	0.0
Gross Unrealized Losses	0.0	0.0
Estimated Fair Value	1.2	1.2
Private mortgage-backed securities
Available-for-Sale:
Amortized Cost	31.0	47.6
Gross Unrealized Gains	0.2	0.0
Gross Unrealized Losses	(0.1)	(0.4)
Estimated Fair Value	31.1	47.2
Corporate securities
Available-for-Sale:
Amortized Cost	271.9	134.5
Gross Unrealized Gains	4.8	1.2
Gross Unrealized Losses	(0.8)	0.0
Estimated Fair Value	275.9	135.7
Held-to-Maturity:
Amortized Cost	4.0	13.9
Gross Unrealized Gains	0.2	0.1
Gross Unrealized Losses	0.0	0.0
Estimated Fair Value	4.2	14.0
Other investments
Available-for-Sale:
Amortized Cost	1.2	3.2
Gross Unrealized Gains	0.0	0.0
Gross Unrealized Losses	0.0	0.0
Estimated Fair Value	1.2	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