CONSOLIDATED STATEMENTS OF CASH FLOWS (Unaudited) - USD ($) $ in Millions	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017	Dec. 31, 2016
Net Cash Provided by (Used in) Operating Activities, Continuing Operations [Abstract]
Net (loss) income	$ 30.6	$ 551.5	$ (50.9)	$ 581.1
Charges (credits) to reconcile net income to net cash used for operations:
Net realized and unrealized investment gains	(66.1)	(32.5)	(22.9)	(102.5)
Deferred income benefit			(4.7)	(6.9)
Net income from discontinued operations	0.0	15.2	0.0	(20.5)
Net loss (gain) from sale of discontinued operations, net of tax		(554.3)	17.2	(552.7)
Amortization of restricted share and option awards			9.7	10.1
Amortization and depreciation			18.4	15.8
Other operating items:
Net change in unearned insurance premiums			19.4	36.2
Net change in deferred acquisition costs			(2.8)	(3.5)
Net change in restricted cash			(12.1)	0.0
Net change in other assets and liabilities, net			(1.8)	(21.5)
Net cash used for operations - continuing operations			(30.5)	(64.4)
Net cash (used for) provided from operations			(30.5)	92.6
Cash flows from investing activities:
Net change in short-term investments			(138.5)	(612.2)
Sales of fixed maturity and convertible investments			1,768.5	1,631.2
Maturities, calls and paydowns of fixed maturity and convertible investments			124.4	178.6
Sales of common equity securities			129.8	407.8
Distributions and redemptions of other long-term investments and settlements of forward contracts			(5.5)	1.3
Sales of unconsolidated affiliates and consolidated subsidiaries, net of cash sold			0.0	1,131.0
Net settlement of investment cash flows and contributions with discontinued operations			0.0	167.7
Purchases of other long-term investments			(65.9)	(67.7)
Purchases of common equity securities			(253.0)	(818.6)
Purchases of fixed maturity and convertible investments			(833.1)	(1,189.8)
Purchases of consolidated subsidiaries, net of cash acquired of $71.9			(252.1)	0.0
Other investing activities, net			33.8	(134.6)
Net cash provided from investing activities - continuing operations			508.4	694.7
Net cash provided from investing activities			508.4	697.7
Cash flows from financing activities:
Draw down of debt and revolving line of credit			50.9	361.0
Repayment of debt and revolving line of credit			(11.2)	(364.6)
Cash dividends paid to the Company’s common shareholders			(3.8)	(4.6)
Common shares repurchased			(504.7)	(714.6)
Distribution to non-controlling interest shareholders			(4.6)	(0.9)
Contributions from discontinued operations			0.0	45.2
Payments on contingent consideration earnout liability			(2.6)	0.0
Capital contributions from BAM members			26.9	25.7
Acquisition of additional subsidiary shares from non-controlling interest			(1.7)	(0.7)
Fidus Re premium payment			(3.0)	0.0
Restricted share statutory withholding tax payments			(8.4)	(9.3)
Net cash used for financing activities - continuing operations			(462.2)	(662.8)
Net cash used for financing activities			(462.2)	(724.7)
Effect of exchange rate changes on cash			(0.6)	0.0
Net change in cash during the period - continuing operations			15.7	(32.5)
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	0.0	$ 0.9
Less: cash held for sale, excluding discontinued operations, at the end of period	0.0	0.0	0.0	0.0	0.0
Cash balances at end of period	112.2	48.6	112.2	48.6	97.1
Supplemental cash flows information:
Interest paid			(5.3)	(0.5)
Net income tax refunds			(0.5)	0.0
Discontinued Operations
Other operating items:
Net cash (used for) provided from operations - discontinued operations			0.0	157.0
Cash flows from investing activities:
Net cash provided from investing activities - discontinued operations			0.0	3.0
Cash flows from financing activities:
Net cash used for financing activities - discontinued operations			0.0	(61.9)
Cash balances at beginning of period			0.0	70.5	70.5
Cash balances at end of period	$ 0.0	$ 71.3	0.0	71.3	$ 0.0
Supplemental cash flows information:
Net income tax refunds			$ 0.0	$ 0.0