Consolidated Statements of Cash Flows - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Cash flows from operating activities:
Net income	$ 21.0	$ 51.2	$ 107.4
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization, net	14.6	15.7	11.7
Share-based compensation	3.7	4.5	3.6
Net realized gain on investments	(36.5)	(24.3)	(20.7)
Changes in operating assets and liabilities:
Deferred policy acquisition benefits	(0.7)	(2.6)	(29.7)
Accrued investment income and other assets	(4.5)	(2.4)	0.2
Postretirement and pension benefits	(28.8)	(6.0)	(7.3)
Reinsurance recoverable on losses and loss expenses payable and prepaid reinsurance premiums	3.0	3.0	(1.9)
Other liabilities and due to/from affiliates, net	7.3	(39.9)	(50.8)
Losses and loss expenses payable	128.6	69.8	23.3
Unearned premiums	1.5	3.9	121.4
Excess tax expense on share-based awards	(0.2)	(0.3)	0.0
Federal income taxes	4.5	13.7	(81.6)
Cash provided from December 31, 2014 unearned premium transfer related to the homeowners quota-share reinsurance arrangement	0.0	63.5	0.0
Net cash provided by operating activities	113.5	149.8	75.6
Cash flows from investing activities:
Purchases of fixed maturities available-for-sale	(589.3)	(573.9)	(431.4)
Purchases of equity securities available-for-sale	(143.1)	(154.0)	(119.0)
Purchases of other invested assets	(1.5)	(6.9)	(1.8)
Maturities, calls and pay downs of fixed maturities available-for-sale	240.9	241.0	232.4
Sales of fixed maturities available-for-sale	211.6	180.7	163.5
Sales of equity securities available-for-sale	153.0	144.8	99.2
Sales of other invested assets available-for-sale	0.8	0.7	0.6
Net disposals of property and equipment	0.0	(0.1)	0.0
Net cash used in investing activities	(127.6)	(167.7)	(56.5)
Cash flows from financing activities:
Proceeds from issuance of common stock	2.2	6.2	3.5
Payments to acquire treasury stock	(0.2)	(0.3)	(0.1)
Payments of dividends (affiliates $10.4, $10.4 and $10.2, respectively)	(16.6)	(16.5)	(16.5)
Excess tax expense on share-based awards	0.2	0.3	0.0
Proceeds from long-term debt	21.5	0.0	0.0
Net cash provided by (used in) financing activities	7.1	(10.3)	(13.1)
Net (decrease) increase in cash and cash equivalents	(7.0)	(28.2)	6.0
Cash and cash equivalents at beginning of year	58.1	86.3	80.3
Cash and cash equivalents at end of year	51.1	58.1	86.3
Supplemental disclosures:
Interest paid (affiliates $0.8, $0.7 and $0.7, respectively)	5.4	5.3	5.2
Federal income taxes paid	$ 0.0	$ 6.4	$ 1.0