Consolidated Statements of Cash Flows - USD ($) $ in Millions		9 Months Ended
		Sep. 30, 2017	Sep. 30, 2016
Statement of Cash Flows [Abstract]
Net income		$ 1,017.8	$ 659.1
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation		126.7	94.6
Amortization of intangible assets		48.2	46.6
Net amortization of fixed-income securities		67.5	57.1
Amortization of equity-based compensation		76.6	64.6
Net realized (gains) losses on securities		(59.3)	(29.0)
Net (gains) losses on disposition of property and equipment		5.3	4.1
(Gains) losses on extinguishment of debt		(0.2)	(1.6)
Net loss on exchange transaction		0.0	4.5
Changes in:
Premiums receivable		(1,010.3)	(752.8)
Reinsurance recoverables		(816.1)	(405.1)
Prepaid reinsurance premiums		(41.2)	44.9
Deferred acquisition costs		(131.4)	(128.0)
Income taxes		(111.2)	(121.2)
Unearned premiums		1,536.7	1,154.6
Loss and loss adjustment expense reserves		1,985.1	1,183.2
Accounts payable, accrued expenses, and other liabilities		622.1	464.8
Restricted cash		(16.5)	(22.0)
Other, net		(93.2)	(42.6)
Net cash provided by operating activities		3,206.6	2,275.8
Purchases:
Fixed maturities		(9,623.6)	(7,364.6)
Equity securities		(155.2)	(367.1)
Sales:
Fixed maturities		3,431.7	4,746.1
Equity securities		150.6	278.2
Maturities, paydowns, calls, and other:
Fixed maturities		3,872.5	4,189.0
Equity securities		50.0	0.0
Net sales (purchases) of short-term investments		(721.8)	(3,665.2)
Net unsettled security transactions		210.5	208.4
Purchases of property and equipment		(109.7)	(162.1)
Sales of property and equipment		13.8	4.5
Net cash disposed in exchange transaction1	[1]	0.0	(7.7)
Acquisition of an insurance company, net of cash acquired		(18.1)	0.0
Net cash used in investing activities		(2,899.3)	(2,140.5)
Cash Flows From Financing Activities
Proceeds from exercise of equity options		0.5	0.0
Tax benefit from vesting of equity-based compensation		0.0	8.0
Net proceeds from debt issuance		841.1	495.6
Payments of debt		(42.8)	(19.2)
Redemption/reacquisition of subordinated debt		(635.6)	(18.2)
Dividends paid to shareholders		(395.4)	(519.0)
Acquisition of treasury shares for restricted stock tax liabilities		(57.2)	(24.7)
Acquisition of treasury shares acquired in open market		(4.9)	(138.9)
Net cash used in financing activities		(294.3)	(216.4)
Effect of exchange rate changes on cash		0.4	0.4
Increase (decrease) in cash		13.4	(80.7)
Cash, January 1		211.5	224.1
Cash, September 30		$ 224.9	$ 143.4

[1] See Note 7 – Supplemental Cash Flow Information for further discussion.