CONSOLIDATED STATEMENTS OF CASH FLOWS (Unaudited) - USD ($) $ in Millions	3 Months Ended			12 Months Ended
	Mar. 31, 2018	Dec. 31, 2017	Mar. 31, 2017	Dec. 31, 2017	Dec. 31, 2016
Net Cash Provided by (Used in) Operating Activities, Continuing Operations [Abstract]
Net loss (income)	$ (66.6)		$ 28.0
Charges (credits) to reconcile net income to net cash used for operations:
Net realized and unrealized investment losses	53.7		(36.3)
Deferred income benefit	(0.8)		(3.0)
Net income from discontinued operations	0.0		(32.3)
Net (gain) loss from sale of discontinued operations, net of tax	(0.1)	$ (0.1)	1.0
Amortization of restricted share and option awards	3.2		1.8
Amortization and depreciation	4.8		5.8
Net change in unearned insurance premiums	3.4		17.4
Net change in deferred acquisition costs	(1.1)		(1.3)
Net change in other assets and liabilities, net	(41.3)		(35.9)
Net cash used for operations - continuing operations	(44.8)		(54.8)
Net cash provided from used for operations	(44.7)		(56.1)
Cash flows from investing activities:
Net change in short-term investments	(587.5)		36.1
Sales of fixed maturity and convertible investments	1,266.5		375.4
Maturities, calls and paydowns of fixed maturity and convertible investments	40.6		70.2
Sales of common equity securities	14.4		15.9
Distributions and redemptions of other long-term investments and settlements of forward contracts	(6.9)		3.2
Net settlement of investment cash flows and contributions with discontinued operations	0.1		0.0
Purchases of other long-term investments	(46.0)		(21.6)
Purchases of common equity securities	(109.4)		(111.9)
Purchases of fixed maturity and convertible investments	(537.1)		(363.4)
Net change in unsettled investment purchases and sales	42.9		3.1
Net acquisitions of property and equipment	(0.1)		0.0
Net cash provided from investing activities - continuing operations	77.5		7.0
Net cash provided from investing activities	77.4		39.3
Cash flows from financing activities:
Repayment of debt and revolving line of credit	(2.3)		(1.2)
Cash dividends paid to the Company’s common shareholders	(3.8)		(4.6)
Distribution to non-controlling interest shareholders	0.0		(0.3)
Contributions from discontinued operations	0.0		15.1
Capital contributions from BAM members	4.9		9.6
Restricted share statutory withholding tax payments	(8.4)		(6.5)
Net cash used for financing activities - continuing operations	(9.6)		12.1
Net cash used for financing activities	(9.6)		(8.9)
Net change in cash during the period - continuing operations	23.1		(35.7)
Cash balances at beginning of period	97.1		80.2	$ 80.2
Add: cash held for sale, excluding discontinued operations, at the beginning of period	0.0	0.0	0.0	0.0	$ 0.9
Less: cash held for sale, excluding discontinued operations, at the end of period	0.0	0.0	0.0	0.0	0.9
Cash balances at end of period	120.2	97.1	45.4	97.1	80.2
Supplemental cash flows information:
Interest paid	(0.3)		(0.2)
Net income tax refunds	0.2		0.0
Discontinued Operations
Charges (credits) to reconcile net income to net cash used for operations:
Net cash provided from (used for) operations - discontinued operations	0.1		(1.3)
Cash flows from investing activities:
Net cash (used for) provided from investing activities - discontinued operations	(0.1)		32.3
Cash flows from financing activities:
Net cash used for financing activities - discontinued operations	0.0		(21.0)
Cash balances at beginning of period	0.0		70.5	70.5
Cash balances at end of period	0.0	$ 0.0	79.6	$ 0.0	$ 70.5
Supplemental cash flows information:
Net income tax refunds	$ 0.0		$ 0.0