Income Taxes (Effective Rate Reconciliation) (Details) - USD ($) $ in Millions			3 Months Ended								12 Months Ended
	Dec. 22, 2017	Dec. 21, 2017	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Income Tax Contingency [Line Items]
U.S. statutory rate (as a percent)	21.00%	35.00%									21.00%	35.00%
Tax benefit (expense) at the U.S. statutory rate											$ 37.4	$ (2.7)	$ 51.6
Differences in taxes resulting from:
Change in valuation allowance											(31.0)	42.6	6.9
State taxes											4.0	0.6	(1.2)
Non-U.S. earnings, net of foreign taxes											(2.9)	21.5	(19.2)
Withholding tax											(2.7)	(2.0)	(0.2)
Member’s surplus contributions (“MSC”)											(2.6)	(3.0)	(2.3)
Tax rate changes											1.7	(44.3)	(3.9)
Tax reserve adjustments											(0.8)	(0.3)	0.0
Tax exempt interest and dividends											0.6	0.5	0.1
Officer compensation											0.0	(4.1)	0.0
Other, net											0.3	(1.0)	1.1
Income tax (expense) benefit			$ 3.6	$ 3.6	$ (2.5)	$ (0.7)	$ 2.5	$ 4.0	$ 1.0	$ 0.3	4.0	7.8	32.9
Pre-tax loss			$ (144.1)	$ 44.3	$ (12.4)	$ (66.0)	$ 5.2	$ 8.4	$ (2.2)	$ (3.6)	(178.2)	7.8	(147.3)
Tax Payments and Receipts
Net income tax payments to (receipts from) national governments, total											3.5	2.0	0.3
Non-US
Differences in taxes resulting from:
Pre-tax loss											$ (30.1)	$ 71.3	$ (66.3)