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Income Taxes (Effective Rate Reconciliation) (Details) - USD ($) $ in Millions			3 Months Ended								12 Months Ended
	Dec. 22, 2017	Dec. 21, 2017	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Income Tax Contingency [Line Items]
U.S. statutory rate (as a percent)	21.00%	35.00%									21.00%	35.00%
Tax (expense) benefit at the U.S. statutory rate											$ (85.1)	$ 37.4	$ (2.7)
Differences in taxes resulting from:
Change in valuation allowance											63.6	(31.0)	42.6
Non-U.S. earnings, net of foreign taxes											27.1	(2.9)	21.5
State taxes											(17.5)	4.0	0.6
Tax rate changes											(5.7)	1.7	(44.3)
Member’s surplus contributions											(3.6)	(2.6)	(3.0)
Withholding tax											(1.6)	(2.7)	(2.0)
Tax exempt interest and dividends											1.1	0.6	0.5
Tax reserve adjustments											(0.7)	(0.8)	(0.3)
Officer compensation											0.0	0.0	(4.1)
Other, net											(6.9)	0.3	(1.0)
Total income tax (expense) benefit			$ (10.4)	$ (8.8)	$ 0.1	$ (10.2)	$ 3.6	$ 3.6	$ (2.5)	$ (0.7)	(29.3)	4.0	7.8
Pre-tax income (loss)			$ 56.5	$ 51.1	$ 15.8	$ 281.7	$ (144.1)	$ 44.3	$ (12.4)	$ (66.0)	405.1	(178.2)	7.8
Tax Payments and Receipts
Net income tax payments to (receipts from) national governments, total											3.7	3.5	2.0
Non-US
Differences in taxes resulting from:
Pre-tax income (loss)											$ 100.4	$ (30.1)	$ 71.3

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