New Accounting Standards (Details) - USD ($) $ in Millions		12 Months Ended
	Jan. 01, 2020	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Allowance for credit loss	$ 196.0	$ 363.0	$ 196.0	$ 180.0	$ 175.0
Financing receivable, allowance for credit loss, period increase (decrease)	105.3	167.2
Stockholders' equity attributable to parent		4,959.0	4,883.0	4,608.0	4,409.0
Accounting Standards Update 2016-13
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Allowance for credit loss	301.0
Retained Earnings
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Stockholders' equity attributable to parent		$ 869.0	$ 798.0	$ 576.0	$ 368.0
Retained Earnings | Accounting Standards Update 2016-13
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Stockholders' equity attributable to parent	(51.0)
Financing Receivables, Originated Loans And Leases | Accounting Standards Update 2016-13
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Financing receivable, allowance for credit loss, period increase (decrease)	55.0
Longer Duration Loans and Gross Up | Accounting Standards Update 2016-13
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Financing receivable, allowance for credit loss, period increase (decrease)	50.0
Unfunded Loan Commitment | Accounting Standards Update 2016-13
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Allowance for credit loss	10.0
Financing receivable, allowance for credit loss, period increase (decrease)	$ 10.0