Derivative Financial Instruments - Interest Rate Risk (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Mar. 31, 2020
Derivative [Line Items]
Unrealized gains on terminated cash flow hedges remaining in AOCI, before tax	$ 179.0
Unrealized gains on terminated cash flow hedges remaining in AOCI, net of tax	$ 129.3
Remaining term over which unrealized gains will be reclassified into earnings	4 years
Interest rate swaps | Derivatives designated as hedging instruments
Derivative [Line Items]
Net asset fair value		$ 227.5
Notional balances		$ 5,000.0