Consolidated Statements Of Cash Flows - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
CASH FLOWS PROVIDED BY OPERATING ACTIVITIES:
Net income	$ 255.3	$ 83.9	$ 490.3
Adjustments to reconcile net income to cash provided by operating activities:
Net realized gains on sales of investments	(38.1)	(73.9)	(145.0)
Mark to market adjustments	12.7	189.8	22.0
Stock compensation expense	16.4	15.8	14.2
Undistributed income of equity method investments	30.6	22.6	13.9
Changes in:
Reserve for losses and loss expenses, net of reinsurance recoverables	38.1	175.9	8.9
Unearned premiums, net of prepaid reinsurance	(89.2)	(43.9)	139.3
Insurance balances receivable	(39.4)	37.0	(76.8)
Reinsurance recoverable on paid losses	(7.9)	(10.4)	(8.5)
Funds held	174.0	83.2	(91.6)
Reinsurance balances payable	9.0	(5.6)	7.0
Net deferred acquisition costs	44.1	17.0	(24.9)
Net deferred tax assets	(14.3)	(2.7)	4.2
Accounts payable and accrued liabilities	(4.9)	(10.1)	8.0
Other items, net	12.9	34.2	55.9
Net cash provided by operating activities	399.3	512.8	416.9
CASH FLOWS PROVIDED BY (USED IN) INVESTING ACTIVITIES:
Purchases of trading securities	(5,423.7)	(5,863.2)	(7,630.0)
Purchases of other invested assets	(119.0)	(126.7)	(307.9)
Sales of trading securities	5,929.9	5,328.8	7,536.9
Sales of other invested assets	96.3	161.3	267.9
Purchases of fixed assets	(3.5)	(31.8)	(59.7)
Net cash paid for acquisitions	(1.2)	(124.4)	(2.6)
Change in restricted cash	(15.9)	20.3	68.4
Net cash provided by (used in) investing activities	462.9	(635.7)	(127.0)
CASH FLOWS (USED IN) PROVIDED BY FINANCING ACTIVITIES:
Dividends paid	(91.5)	(114.1)	(76.7)
Share repurchases	(166.3)	(246.4)	(175.9)
Repayment of senior notes	(500.0)	0.0	0.0
Proceeds from the exercise of stock options	10.3	10.1	10.0
Proceeds from senior notes	0.0	496.7	0.0
Proceeds from other long-term debt	0.0	4.0	19.2
Repayment of other long-term debt	(0.3)	(0.2)	0.0
Net cash (used in) provided by used in financing activities	(747.8)	150.1	(223.4)
Effect of exchange rate changes on foreign currency cash	(1.5)	(8.5)	(9.1)
NET INCREASE IN CASH AND CASH EQUIVALENTS	112.9	18.7	57.4
CASH AND CASH EQUIVALENTS, BEGINNING OF PERIOD	608.0	589.3	531.9
CASH AND CASH EQUIVALENTS, END OF PERIOD	720.9	608.0	589.3
Supplemental disclosure of cash flow information:
Cash paid for income taxes	3.5	2.5	18.4
Cash paid for interest expense	$ 75.8	$ 54.0	$ 54.0