Segments - Operating Segment Information (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020	Mar. 31, 2021	Dec. 31, 2020	Mar. 31, 2020	Jan. 01, 2020	Dec. 31, 2019
Operating Statistics [Line Items]
Service charges and fees	$ 7.4	$ 5.1	$ 14.1	$ 11.5
Cash, Cash Equivalents and Investment Securities	11,240.8		11,240.8			$ 8,176.5
Loans Receivable Held-for-sale, Amount	4,465.2		4,465.2			0.0
Loans and Leases Receivable, Net of Deferred Income	30,026.4		30,026.4			27,053.0
Less: Allowance for credit losses	(232.9)		(232.9)			(278.9)
Loans, net	29,793.5		29,793.5			26,774.1
Other Repossessed Assets	3.9		3.9			1.4
Goodwill and intangible assets, net	610.7		610.7			298.5
Other Assets Segment	2,954.9		2,954.9			1,210.5
Assets	49,069.0		49,069.0			36,461.0
Deposits	41,921.0		41,921.0			31,930.5
Other borrowings	1,735.2		1,735.2			553.7
Other Liabilities Segment	1,378.3		1,378.3			563.3
Total liabilities	45,034.5		45,034.5			33,047.5
Stockholders' equity	4,034.5	3,102.4	4,034.5	3,102.4	$ 3,712.7	3,413.5	$ 2,999.6	$ 2,991.8	$ 3,016.7
Liabilities and Equity	49,069.0		49,069.0			36,461.0
Excess Funds Provided by (Used in) Segment Basis	0.0		0.0			0.0
Net interest income	370.5	298.4	687.8	567.4
Provision for Loan, Lease, and Other Losses	(14.5)	(92.0)	(46.9)	(143.2)
Net interest income (loss) after provision for credit losses	385.0	206.4	734.7	424.2
Noninterest Income	136.0	21.3	155.7	26.4
Non-interest expense	(244.8)	(114.8)	(379.8)	(235.3)
Income (Loss) from Continuing Operations before Equity Method Investments, Income Taxes, Noncontrolling Interest	276.2	112.9	510.6	215.3
Income tax expense (benefit)	52.4	19.6	94.3	38.1
Net income	223.8	93.3	416.3	177.2
Debit and credit card interchange fees	1.6	0.9	2.9	2.3
Success Fees	0.0	0.3	1.0	0.4
Other non-interest income	0.2	0.1	0.4	0.2
Contract with Customer, Liability, Revenue Recognized	9.2	6.4	18.4	14.4
Other revenue	126.8	14.9	20.8	12.0
Operating Segments [Member] | Commercial Segment
Operating Statistics [Line Items]
Service charges and fees	6.9	4.8	13.2	10.9
Cash, Cash Equivalents and Investment Securities	13.4		13.4			12.0
Loans Receivable Held-for-sale, Amount	0.0		0.0
Loans and Leases Receivable, Net of Deferred Income	20,642.9		20,642.9			20,245.8
Less: Allowance for credit losses	212.1		212.1			263.4
Loans, net	20,430.8		20,430.8			19,982.4
Other Repossessed Assets	3.9		3.9			1.4
Goodwill and intangible assets, net	295.4		295.4			296.1
Other Assets Segment	254.3		254.3			257.0
Assets	20,997.8		20,997.8			20,548.9
Deposits	26,262.3		26,262.3			21,448.0
Other borrowings	0.0		0.0			0.0
Other Liabilities Segment	302.9		302.9			170.4
Total liabilities	26,565.2		26,565.2			21,618.4
Stockholders' equity	2,178.7		2,178.7			1,992.2
Liabilities and Equity	28,743.9		28,743.9			23,610.6
Excess Funds Provided by (Used in) Segment Basis	7,746.1		7,746.1			3,061.7
Net interest income	280.7	254.9	544.5	483.4
Provision for Loan, Lease, and Other Losses	(18.6)	(98.0)	(54.8)	(145.8)
Net interest income (loss) after provision for credit losses	299.3	156.9	599.3	337.6
Noninterest Income	13.9	9.6	33.1	21.0
Non-interest expense	(104.1)	(71.2)	(202.4)	(153.6)
Income (Loss) from Continuing Operations before Equity Method Investments, Income Taxes, Noncontrolling Interest	209.1	95.3	430.0	205.0
Income tax expense (benefit)	50.4	22.9	103.2	48.9
Net income	158.7	72.4	326.8	156.1
Debit and credit card interchange fees	1.6	0.9	2.9	2.3
Success Fees	0.0	0.3	1.0	0.4
Other non-interest income	0.2	0.1	0.4	0.2
Contract with Customer, Liability, Revenue Recognized	8.7	6.1	17.5	13.8
Other revenue	5.2	3.5	15.6	7.2
Operating Segments [Member] | Consumer Related Segment
Operating Statistics [Line Items]
Service charges and fees	0.5	0.3	0.9	0.6
Cash, Cash Equivalents and Investment Securities	79.5		79.5			45.6
Loans Receivable Held-for-sale, Amount	4,465.2		4,465.2
Loans and Leases Receivable, Net of Deferred Income	9,387.0		9,387.0			6,798.2
Less: Allowance for credit losses	20.8		20.8			15.4
Loans, net	9,366.2		9,366.2			6,782.8
Other Repossessed Assets	0.0		0.0			0.0
Goodwill and intangible assets, net	315.3		315.3			2.4
Other Assets Segment	1,165.4		1,165.4			96.6
Assets	15,391.6		15,391.6			6,927.4
Deposits	14,841.8		14,841.8			9,936.8
Other borrowings	355.8		355.8			0.0
Other Liabilities Segment	143.9		143.9			3.3
Total liabilities	15,341.5		15,341.5			9,940.1
Stockholders' equity	1,348.7		1,348.7			579.1
Liabilities and Equity	16,690.2		16,690.2			10,519.2
Excess Funds Provided by (Used in) Segment Basis	1,298.6		1,298.6			3,591.8
Net interest income	139.5	72.4	247.5	128.1
Provision for Loan, Lease, and Other Losses	(7.2)	(10.5)	(8.9)	(7.5)
Net interest income (loss) after provision for credit losses	132.3	82.9	238.6	135.6
Noninterest Income	116.9	0.4	117.4	0.8
Non-interest expense	(135.6)	(21.2)	(170.9)	(47.0)
Income (Loss) from Continuing Operations before Equity Method Investments, Income Taxes, Noncontrolling Interest	113.6	62.1	185.1	89.4
Income tax expense (benefit)	27.5	14.7	44.9	21.1
Net income	86.1	47.4	140.2	68.3
Debit and credit card interchange fees	0.0	0.0	0.0	0.0
Success Fees	0.0	0.0	0.0	0.0
Other non-interest income	0.0	0.0	0.0	0.0
Contract with Customer, Liability, Revenue Recognized	0.5	0.3	0.9	0.6
Other revenue	116.4	0.1	0.0	0.2
Corporate & Other
Operating Statistics [Line Items]
Service charges and fees	0.0	0.0	0.0	0.0
Cash, Cash Equivalents and Investment Securities	11,147.9		11,147.9			8,118.9
Loans Receivable Held-for-sale, Amount	0.0		0.0
Loans and Leases Receivable, Net of Deferred Income	(3.5)		(3.5)			9.0
Less: Allowance for credit losses	0.0		0.0			0.1
Loans, net	(3.5)		(3.5)			8.9
Other Repossessed Assets	0.0		0.0			0.0
Goodwill and intangible assets, net	0.0		0.0			0.0
Other Assets Segment	1,535.2		1,535.2			856.9
Assets	12,679.6		12,679.6			8,984.7
Deposits	816.9		816.9			545.7
Other borrowings	1,379.4		1,379.4			553.7
Other Liabilities Segment	931.5		931.5			389.6
Total liabilities	3,127.8		3,127.8			1,489.0
Stockholders' equity	507.1		507.1			842.2
Liabilities and Equity	3,634.9		3,634.9			2,331.2
Excess Funds Provided by (Used in) Segment Basis	(9,044.7)		(9,044.7)			$ (6,653.5)
Net interest income	(49.7)	(28.9)	(104.2)	(44.1)
Provision for Loan, Lease, and Other Losses	3.1	(4.5)	1.0	(4.9)
Net interest income (loss) after provision for credit losses	(46.6)	(33.4)	(103.2)	(49.0)
Noninterest Income	5.2	11.3	5.2	4.6
Non-interest expense	(5.1)	(22.4)	(6.5)	(34.7)
Income (Loss) from Continuing Operations before Equity Method Investments, Income Taxes, Noncontrolling Interest	(46.5)	(44.5)	(104.5)	(79.1)
Income tax expense (benefit)	(25.5)	(18.0)	(53.8)	(31.9)
Net income	(21.0)	(26.5)	(50.7)	(47.2)
Debit and credit card interchange fees	0.0	0.0	0.0	0.0
Success Fees	0.0	0.0	0.0	0.0
Other non-interest income	0.0	0.0	0.0	0.0
Contract with Customer, Liability, Revenue Recognized	0.0	0.0	0.0	0.0
Other revenue	$ 5.2	$ 11.3	$ 5.2	$ 4.6