Income Taxes - Effective Rate Reconciliation (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Income Tax Contingency [Line Items]
U.S. statutory rate (as a percent)									21.00%	21.00%	21.00%
Tax (expense) benefit at the U.S. statutory rate									$ (135.5)	$ (85.1)	$ 37.4
Differences in taxes resulting from:
Reorganization									130.5	0.0	0.0
Non-U.S. earnings, net of foreign taxes									78.4	27.1	(2.9)
Change in valuation allowance									(29.2)	63.6	(31.0)
State taxes									(8.5)	(17.5)	4.0
Withholding tax									(5.0)	(1.6)	(2.7)
Member’s surplus contributions									(4.8)	(3.6)	(2.6)
Tax rate changes									2.7	(5.7)	1.7
Tax reserve adjustments									1.9	(0.7)	(0.8)
Officer compensation									(1.1)	0.0	0.0
Tax exempt interest and dividends									0.8	1.1	0.6
Other, net									(9.7)	(6.9)	0.3
Total income tax benefit (expense)	$ 117.6	$ (98.5)	$ (24.1)	$ 25.5	$ (10.4)	$ (8.8)	$ 0.1	$ (10.2)	20.5	(29.3)	4.0
Pre-tax income (loss)	$ 357.3	$ 321.2	$ 132.7	$ (166.0)	$ 56.5	$ 51.1	$ 15.8	$ 281.7	645.2	405.1	(178.2)
Tax Payments and Receipts
Net income tax payments to US									16.2	3.7	3.5
Non-US
Differences in taxes resulting from:
Pre-tax income (loss)									$ 327.8	$ 100.4	$ (30.1)