Loans Receivable (Summary of Changes in Expected Future Net Cash Flows) (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2020	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Accounts, Notes, Loans and Financing Receivable [Line Items]
Balance, beginning of period	$ 9,005.2	$ 9,005.2	$ 7,828.8	$ 6,285.1
New Consumer Loan Assignments		5,131.6	5,305.0	5,074.4
Realized Net Cash Flows		(4,552.9)	(4,152.2)	(3,580.8)
Forecast changes	(206.5)	(46.3)	14.6	42.3
Transfers		6.8	9.0	7.8
Balance, end of period		9,544.4	9,005.2	7,828.8
Dealer Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Balance, beginning of period	5,577.0	5,577.0	5,045.9	4,240.7
New Consumer Loan Assignments		3,113.4	3,396.1	3,405.0
Realized Net Cash Flows		(2,870.6)	(2,749.7)	(2,507.8)
Forecast changes		(41.1)	(7.9)	2.0
Transfers		(114.4)	(107.4)	(94.0)
Balance, end of period		5,664.3	5,577.0	5,045.9
Purchased Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Balance, beginning of period	$ 3,428.2	3,428.2	2,782.9	2,044.4
New Consumer Loan Assignments		2,018.2	1,908.9	1,669.4
Realized Net Cash Flows		(1,682.3)	(1,402.5)	(1,073.0)
Forecast changes		(5.2)	22.5	40.3
Transfers		121.2	116.4	101.8
Balance, end of period		$ 3,880.1	$ 3,428.2	$ 2,782.9