Consolidated Statements of Cash Flows - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Statement of Cash Flows [Abstract]
Special dividend paid in September 2019	$ 303.4
Special dividend declared in December 2017 and paid in January 2018		$ 269.2
Cash flows from operating activities:
Net income (loss)	1,056.4	370.5	$ 560.5
Adjustments to reconcile net income (loss) to net cash provided by operating activities:
Deferred policy acquisition costs	(8.9)	(18.4)	(23.3)
Premiums and other receivables	32.5	(29.3)	(79.3)
Unpaid claims and related items	214.6	148.4	(123.7)
Unearned premiums and other policyholders' liabilities	32.2	95.1	152.7
Income taxes	37.2	(69.5)	49.5
Prepaid federal income taxes	129.8	(15.5)	(31.8)
Reinsurance balances and funds	(9.7)	13.5	25.3
Realized from actual transactions and impairments	36.6	58.2	211.6
Unrealized from changes in fair value of equity securities	(599.5)	293.8	0.0
Accounts payable, accrued expenses and other	88.0	30.0	134.5
Total	936.2	760.5	452.8
Fixed maturity securities:
Maturities and early calls	779.0	964.0	1,000.6
Sales	663.1	299.1	468.4
Sales of:
Equity securities	809.9	402.6	698.5
Other - net	33.0	19.4	30.0
Purchases of:
Fixed maturity securities:	(1,702.1)	(1,421.9)	(1,607.2)
Payments to Acquire Held-to-maturity Securities	0.0	0.0	(114.5)
Equity securities	(815.6)	(752.5)	(727.2)
Other - net	(60.9)	(51.6)	(54.3)
Purchase of a business	(1.2)	(13.1)	0.0
Net decrease (increase) in short-term investments	(129.7)	314.2	11.8
Other - net	0.0	0.1	(0.1)
Total	(424.6)	(239.5)	(293.9)
Cash flows from financing activities:
Issuance of common shares	13.8	13.1	17.8
Redemption of debentures and notes	(8.4)	(4.7)	(3.9)
Origination of Loans to Employee Stock Ownership Plans	0.0	(50.0)	0.0
Dividends on common shares (including a special dividend declared in December 2017 and paid in January 2018 of $269.2)	(538.7)	(498.8)	(198.8)
Other - net	0.2	(6.0)	6.4
Total	(533.1)	(546.5)	(178.5)
Increase (decrease) in cash:	(21.4)	(25.6)	(19.7)
Cash, beginning of year	100.3	125.9	145.7
Cash, end of year	78.8	100.3	125.9
Supplemental cash flow information:
Cash paid (received) during the period for: Interest	42.1	50.8	62.5
Cash paid (received) during the period for: Income taxes	$ 229.4	$ 137.2	$ 106.3