Derivative Instruments - Gross and Net Information About Financial Instruments Subject to Master Netting Arrangements (Details) - USD ($) $ in Millions	Sep. 30, 2021	Dec. 31, 2020
Freestanding derivative assets
Gross Amounts Recognized	$ 1,141.9	$ 2,219.8
Gross Amounts Offset in the Condensed Consolidated Balance Sheets	0.0	0.0
Net Amounts Presented in the Condensed Consolidated Balance Sheets	1,141.9	2,219.8
Gross Amounts Not Offset in the Condensed Consolidated Balance Sheets, Financial Instruments	40.4	35.1
Gross Amounts Not Offset in the Condensed Consolidated Balance Sheets, Cash Collateral	583.0	1,097.9
Gross Amounts Not Offset in the Condensed Consolidated Balance Sheets, Securities Collateral	470.5	890.0
Net Amount	48.0	196.8
Freestanding derivative liabilities
Gross Amounts Recognized	40.4	56.4
Gross Amounts Offset in the Condensed Consolidated Balance Sheets	0.0	0.0
Fair Value	40.4	56.4
Gross Amounts Not Offset in the Condensed Consolidated Balance Sheets, Financial Instruments	40.4	35.1
Gross Amounts Not Offset in the Condensed Consolidated Balance Sheets, Cash Collateral	0.0	13.1
Gross Amounts Not Offset in the Condensed Consolidated Balance Sheets, Securities Collateral	0.0	0.0
Net Amount	0.0	8.2
Securities loaned
Gross Amounts Recognized	20.7	13.3
Gross Amounts Offset in the Condensed Consolidated Balance Sheets	0.0	0.0
Net Amounts Presented in the Condensed Consolidated Balance Sheets	20.7	13.3
Gross Amounts Not Offset in the Condensed Consolidated Balance Sheets, Financial Instruments	0.0	0.0
Gross Amounts Not Offset in the Condensed Consolidated Balance Sheets, Cash Collateral	20.7	13.3
Gross Amounts Not Offset in the Condensed Consolidated Balance Sheets, Securities Collateral	0.0	0.0
Net Amount	0.0	0.0
Repurchase agreements
Gross Amounts Recognized	306.0	1,100.0
Gross Amounts Offset in the Condensed Consolidated Balance Sheets	0.0	0.0
Net Amounts Presented in the Condensed Consolidated Balance Sheets	306.0	1,100.0
Gross Amounts Not Offset in the Condensed Consolidated Balance Sheets, Financial Instruments	0.0	0.0
Gross Amounts Not Offset in the Condensed Consolidated Balance Sheets, Cash Collateral	0.0	0.0
Gross Amounts Not Offset in the Condensed Consolidated Balance Sheets, Securities Collateral	306.0	1,100.0
Net Amount	0.0	0.0
Total financial liabilities
Gross Amounts Recognized	367.1	1,169.7
Gross Amounts Offset in the Condensed Consolidated Balance Sheets	0.0	0.0
Net Amounts Presented in the Condensed Consolidated Balance Sheets	367.1	1,169.7
Gross Amounts Not Offset in the Condensed Consolidated Balance Sheets, Financial Instruments	40.4	35.1
Gross Amounts Not Offset in the Condensed Consolidated Balance Sheets, Cash Collateral	20.7	26.4
Gross Amounts Not Offset in the Condensed Consolidated Balance Sheets, Securities Collateral	306.0	1,100.0
Net Amount	$ 0.0	$ 8.2