Investment Securities - Gross Unrealized Losses and Fair Values of Investment Securities (Details) - USD ($) $ in Millions	Jun. 30, 2023	Mar. 31, 2023	Dec. 31, 2022	Jun. 30, 2022	Mar. 31, 2022	Dec. 31, 2021
Available-for-Sale:
Fair Value, Less than 12 Months	$ 1,114.2		$ 5,071.9
Gross Unrealized Losses, Less than 12 Months	(67.0)		(346.6)
Fair Value, 12 Months or Longer	4,731.7		1,545.6
Gross Unrealized Losses, 12 Months or Longer	(523.5)		(283.1)
Fair Value, Total	5,845.9		6,617.5
Gross Unrealized Losses, Total	(590.5)		(629.7)
Allowance for Credit Losses	(1.4)	$ (2.6)	0.0	$ 0.0	$ 0.0	$ 0.0
Debt Securities, Available-for-Sale, Amortized Cost, after Allowance for Credit Loss	6,448.2		7,573.5
Gross Unrealized Gains	0.2		2.3
Gross Unrealized Losses	(590.5)		(629.7)
Estimated Fair Value	5,857.9		6,946.1
Debt Securities, Available-for-Sale, Amortized Cost, Excluding Accrued Interest, before Allowance for Credit Loss	6,449.6		7,573.5
Investment securities classified as held to maturity	3,318.4		3,453.7
Allowance for Credit Losses	(0.7)	$ (0.7)	(1.9)	$ (1.6)	$ (1.6)	$ 0.0
Net Amortized Cost	3,317.7		3,451.8
Gross Unrealized Gains	0.4		0.4
Gross Unrealized Losses	(393.4)		(400.0)
Investment debt securities held-to-maturity	2,924.7		3,052.2
US Treasury Notes Securities
Available-for-Sale:
Fair Value, Less than 12 Months	47.2
Gross Unrealized Losses, Less than 12 Months	(2.3)
Fair Value, 12 Months or Longer	171.9
Gross Unrealized Losses, 12 Months or Longer	(28.7)
Fair Value, Total	219.1
Gross Unrealized Losses, Total	(31.0)
U.S. Treasury notes
Available-for-Sale:
Fair Value, Less than 12 Months			168.8
Gross Unrealized Losses, Less than 12 Months			(4.7)
Fair Value, 12 Months or Longer			170.4
Gross Unrealized Losses, 12 Months or Longer			(29.8)
Fair Value, Total			339.2
Gross Unrealized Losses, Total			(34.5)
Allowance for Credit Losses	0.0		0.0
Debt Securities, Available-for-Sale, Amortized Cost, after Allowance for Credit Loss	250.1		675.1
Gross Unrealized Gains	0.0		2.1
Gross Unrealized Losses	(31.0)		(34.5)
Estimated Fair Value	219.1		642.7
Debt Securities, Available-for-Sale, Amortized Cost, Excluding Accrued Interest, before Allowance for Credit Loss	250.1		675.1
Investment securities classified as held to maturity	397.8		396.6
Allowance for Credit Losses	0.0		0.0
Net Amortized Cost	397.8		396.6
Gross Unrealized Gains	0.0		0.0
Gross Unrealized Losses	(8.9)		(10.2)
Investment debt securities held-to-maturity	388.9		386.4
State, county, and municipal securities
Available-for-Sale:
Fair Value, Less than 12 Months	9.0		104.9
Gross Unrealized Losses, Less than 12 Months	(0.4)		(9.0)
Fair Value, 12 Months or Longer	204.0		146.1
Gross Unrealized Losses, 12 Months or Longer	(42.8)		(41.6)
Fair Value, Total	213.0		251.0
Gross Unrealized Losses, Total	(43.2)		(50.6)
Allowance for Credit Losses	0.0		0.0
Debt Securities, Available-for-Sale, Amortized Cost, after Allowance for Credit Loss	258.1		314.3
Gross Unrealized Gains	0.0		0.0
Gross Unrealized Losses	(43.2)		(50.6)
Estimated Fair Value	214.9		263.7
Debt Securities, Available-for-Sale, Amortized Cost, Excluding Accrued Interest, before Allowance for Credit Loss	258.1		314.3
Investment securities classified as held to maturity	180.5		181.2
Allowance for Credit Losses	0.0		(0.1)
Net Amortized Cost	180.5		181.1
Gross Unrealized Gains	0.2		0.2
Gross Unrealized Losses	(27.0)		(31.6)
Investment debt securities held-to-maturity	153.7		149.7
Obligations of U.S. government agencies
Available-for-Sale:
Fair Value, Less than 12 Months	60.6		152.2
Gross Unrealized Losses, Less than 12 Months	(3.1)		(10.1)
Fair Value, 12 Months or Longer	111.0		46.1
Gross Unrealized Losses, 12 Months or Longer	(11.3)		(7.2)
Fair Value, Total	171.6		198.3
Gross Unrealized Losses, Total	(14.4)		(17.3)
Allowance for Credit Losses	0.0		0.0
Debt Securities, Available-for-Sale, Amortized Cost, after Allowance for Credit Loss	186.4		216.2
Gross Unrealized Gains	0.0		0.0
Gross Unrealized Losses	(14.4)		(17.3)
Estimated Fair Value	172.0		198.9
Debt Securities, Available-for-Sale, Amortized Cost, Excluding Accrued Interest, before Allowance for Credit Loss	186.4		216.2
Investment securities classified as held to maturity	353.1		351.7
Allowance for Credit Losses	0.0		0.0
Net Amortized Cost	353.1		351.7
Gross Unrealized Gains	0.0		0.0
Gross Unrealized Losses	(48.9)		(49.6)
Investment debt securities held-to-maturity	304.2		302.1
Private mortgage-backed securities
Available-for-Sale:
Fair Value, Less than 12 Months	0.0		133.1
Gross Unrealized Losses, Less than 12 Months	0.0		(19.1)
Fair Value, 12 Months or Longer	217.1		94.9
Gross Unrealized Losses, 12 Months or Longer	(36.4)		(17.8)
Fair Value, Total	217.1		228.0
Gross Unrealized Losses, Total	(36.4)		(36.9)
Allowance for Credit Losses	0.0		0.0
Debt Securities, Available-for-Sale, Amortized Cost, after Allowance for Credit Loss	253.5		264.9
Gross Unrealized Gains	0.0		0.0
Gross Unrealized Losses	(36.4)		(36.9)
Estimated Fair Value	217.1		228.0
Debt Securities, Available-for-Sale, Amortized Cost, Excluding Accrued Interest, before Allowance for Credit Loss	253.5		264.9
Collateralized Loan Obligations
Available-for-Sale:
Fair Value, Less than 12 Months	0.0		1,082.6
Gross Unrealized Losses, Less than 12 Months	0.0		(33.1)
Fair Value, 12 Months or Longer	1,124.3		28.9
Gross Unrealized Losses, 12 Months or Longer	(17.5)		(0.5)
Fair Value, Total	1,124.3		1,111.5
Gross Unrealized Losses, Total	(17.5)		(33.6)
Allowance for Credit Losses	0.0		0.0
Debt Securities, Available-for-Sale, Amortized Cost, after Allowance for Credit Loss	1,141.8		1,145.2
Gross Unrealized Gains	0.0		0.0
Gross Unrealized Losses	(17.5)		(33.6)
Estimated Fair Value	1,124.3		1,111.6
Debt Securities, Available-for-Sale, Amortized Cost, Excluding Accrued Interest, before Allowance for Credit Loss	1,141.8		1,145.2
Corporate Debt Securities
Available-for-Sale:
Fair Value, Less than 12 Months	16.3		130.6
Gross Unrealized Losses, Less than 12 Months	(0.9)		(8.3)
Fair Value, 12 Months or Longer	211.2		110.8
Gross Unrealized Losses, 12 Months or Longer	(31.7)		(22.5)
Fair Value, Total	227.5		241.4
Gross Unrealized Losses, Total	(32.6)		(30.8)
Allowance for Credit Losses	(1.4)		0.0
Debt Securities, Available-for-Sale, Amortized Cost, after Allowance for Credit Loss	260.1		272.3
Gross Unrealized Gains	0.0		0.0
Gross Unrealized Losses	(32.6)		(30.8)
Estimated Fair Value	227.5		241.5
Debt Securities, Available-for-Sale, Amortized Cost, Excluding Accrued Interest, before Allowance for Credit Loss	261.5		272.3
Investment securities classified as held to maturity	57.1		80.1
Allowance for Credit Losses	(0.7)		(1.8)
Net Amortized Cost	56.4		78.3
Gross Unrealized Gains	0.0		0.0
Gross Unrealized Losses	(7.8)		(7.0)
Investment debt securities held-to-maturity	48.6		71.3
Commercial Mortgage-Backed Securities
Available-for-Sale:
Fair Value, Less than 12 Months	396.0		1,069.8
Gross Unrealized Losses, Less than 12 Months	(23.7)		(59.9)
Fair Value, 12 Months or Longer	696.8		270.8
Gross Unrealized Losses, 12 Months or Longer	(87.0)		(57.0)
Fair Value, Total	1,092.8		1,340.6
Gross Unrealized Losses, Total	(110.7)		(116.9)
Allowance for Credit Losses	0.0		0.0
Debt Securities, Available-for-Sale, Amortized Cost, after Allowance for Credit Loss	1,212.9		1,468.0
Gross Unrealized Gains	0.2		0.2
Gross Unrealized Losses	(110.7)		(117.0)
Estimated Fair Value	1,102.4		1,351.2
Debt Securities, Available-for-Sale, Amortized Cost, Excluding Accrued Interest, before Allowance for Credit Loss	1,212.9		1,468.0
Investment securities classified as held to maturity	517.8		526.8
Allowance for Credit Losses	0.0		0.0
Net Amortized Cost	517.8		526.8
Gross Unrealized Gains	0.0		0.0
Gross Unrealized Losses	(61.4)		(58.3)
Investment debt securities held-to-maturity	456.4		468.5
Residential Mortgage Backed Securities
Available-for-Sale:
Fair Value, Less than 12 Months	272.5		1,213.5
Gross Unrealized Losses, Less than 12 Months	(15.7)		(112.3)
Fair Value, 12 Months or Longer	1,102.8		344.6
Gross Unrealized Losses, 12 Months or Longer	(143.4)		(55.4)
Fair Value, Total	1,375.3		1,558.1
Gross Unrealized Losses, Total	(159.1)		(167.7)
Allowance for Credit Losses	0.0		0.0
Debt Securities, Available-for-Sale, Amortized Cost, after Allowance for Credit Loss	1,534.5		1,726.0
Gross Unrealized Gains	0.0		0.0
Gross Unrealized Losses	(159.1)		(167.7)
Estimated Fair Value	1,375.4		1,558.3
Debt Securities, Available-for-Sale, Amortized Cost, Excluding Accrued Interest, before Allowance for Credit Loss	1,534.5		1,726.0
Investment securities classified as held to maturity	1,311.4		1,391.5
Allowance for Credit Losses	0.0		0.0
Net Amortized Cost	1,311.4		1,391.5
Gross Unrealized Gains	0.0		0.0
Gross Unrealized Losses	(160.7)		(166.6)
Investment debt securities held-to-maturity	1,150.7		1,224.9
Collateralized Mortgage Obligations
Available-for-Sale:
Fair Value, Less than 12 Months	312.6		1,016.4
Gross Unrealized Losses, Less than 12 Months	(20.9)		(90.1)
Fair Value, 12 Months or Longer	892.6		333.0
Gross Unrealized Losses, 12 Months or Longer	(124.7)		(51.3)
Fair Value, Total	1,205.2		1,349.4
Gross Unrealized Losses, Total	(145.6)		(141.4)
Allowance for Credit Losses	0.0		0.0
Debt Securities, Available-for-Sale, Amortized Cost, after Allowance for Credit Loss	1,350.8		1,491.5
Gross Unrealized Gains	0.0		0.0
Gross Unrealized Losses	(145.6)		(141.3)
Estimated Fair Value	1,205.2		1,350.2
Debt Securities, Available-for-Sale, Amortized Cost, Excluding Accrued Interest, before Allowance for Credit Loss	1,350.8		1,491.5
Investment securities classified as held to maturity	500.7		525.8
Allowance for Credit Losses	0.0		0.0
Net Amortized Cost	500.7		525.8
Gross Unrealized Gains	0.2		0.2
Gross Unrealized Losses	(78.7)		(76.7)
Investment debt securities held-to-maturity	422.2		449.3
Other investments
Available-for-Sale:
Allowance for Credit Losses	0.0		0.0
Debt Securities, Available-for-Sale, Amortized Cost, after Allowance for Credit Loss			0.0
Gross Unrealized Gains	0.0		0.0
Gross Unrealized Losses	0.0		0.0
Estimated Fair Value	0.0		0.0
Debt Securities, Available-for-Sale, Amortized Cost, Excluding Accrued Interest, before Allowance for Credit Loss	0.0		0.0
Other Investments
Available-for-Sale:
Investment securities classified as held to maturity			0.0
Allowance for Credit Losses			0.0
Net Amortized Cost			0.0
Gross Unrealized Gains			0.0
Gross Unrealized Losses			0.0
Investment debt securities held-to-maturity			0.0
Other Security Investments
Available-for-Sale:
Fair Value, Less than 12 Months	0.0
Gross Unrealized Losses, Less than 12 Months	0.0
Fair Value, 12 Months or Longer	0.0
Gross Unrealized Losses, 12 Months or Longer	0.0
Fair Value, Total	0.0
Gross Unrealized Losses, Total	$ 0.0
Investment securities classified as held to maturity			$ 0.0