Assets Purchased under Resale Agreements and Sold under Repurchase Agreements (Resale Agreements) (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Offsetting Assets [Line Items]
Gross resale agreements	$ 1,160.0	$ 1,160.0	$ 1,110.0
Loans purchased under agreements to resell, increase	$ 300.0
Loans Purchased Under Resale Agreements
Offsetting Assets [Line Items]
Weighted average yield (as a percent)		2.27%
Securities Purchased Under Resale Agreements
Offsetting Assets [Line Items]
Weighted average yield (as a percent)		1.94%	2.66%	2.63%