Consolidated Statements Of Operations And Comprehensive Income - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
REVENUES:
Gross premiums written	$ 671.7	$ 730.2	$ 800.3	$ 863.5	$ 632.3	$ 754.1	$ 826.0	$ 880.6	$ 3,065.8	$ 3,093.0	$ 2,935.4
Premiums ceded	(226.4)	(201.9)	(222.2)	(159.5)	(167.5)	(147.1)	(222.3)	(108.1)	(810.0)	(645.0)	(613.4)
Net premiums written	445.3	528.3	578.1	704.0	464.8	607.0	603.7	772.5	2,255.8	2,448.0	2,322.0
Change in unearned premiums	131.7	55.7	24.8	(123.9)	158.0	43.7	42.7	(204.0)	88.3	40.4	(139.3)
Net premiums earned	577.0	584.0	602.9	580.1	622.8	650.7	646.4	568.5	2,344.1	2,488.4	2,182.7
Net investment income	58.1	50.6	55.8	53.3	49.1	45.7	42.8	44.5	217.8	182.1	176.9
Net realized investment gains (losses)	(101.9)	10.7	74.5	18.9	(38.8)	(113.6)	(20.2)	45.0	2.1	(127.6)	89.0
Other income	4.8	1.8	5.2	0.6	1.0	0.7	0.9	0.9	12.4	3.5	2.1
Total revenue	538.0	647.1	738.4	652.9	634.1	583.5	669.9	658.9	2,576.4	2,546.4	2,450.7
EXPENSES:
Net losses and loss expenses	387.8	376.3	365.4	372.4	412.7	416.9	431.5	325.2	1,501.8	1,586.3	1,199.2
Acquisition costs	82.0	82.0	87.4	88.3	96.0	100.1	100.6	78.7	339.8	375.4	295.1
General and administrative expenses	106.1	104.2	104.7	96.4	95.0	105.8	108.4	97.1	411.5	406.3	365.7
Other expense	1.8	2.2	1.7	1.1	1.9	1.3	1.2	1.8	6.8	6.2	8.6
Amortization and impairment of intangible assets	3.2	2.5	2.5	2.5	3.7	2.7	2.8	0.6	10.7	9.8	2.5
Interest expense	10.4	13.5	19.9	20.0	18.1	14.5	14.5	14.3	63.7	61.4	57.8
Foreign exchange (gain) loss	0.8	1.0	(2.9)	(3.0)	0.9	(0.8)	1.3	9.9	(4.1)	11.3	1.0
Total expenses	592.1	581.7	578.7	577.7	628.3	640.5	660.3	527.6	2,330.2	2,456.7	1,929.9
Income before income taxes	(54.1)	65.4	159.7	75.2	5.8	(57.0)	9.6	131.3	246.2	89.7	520.8
Income tax (benefit) expense	(13.3)	(3.2)	6.3	1.1	4.0	(5.3)	0.1	7.0	(9.1)	5.8	30.5
NET INCOME	$ (40.8)	$ 68.6	$ 153.4	$ 74.1	$ 1.8	$ (51.7)	$ 9.5	$ 124.3	255.3	83.9	490.3
Other comprehensive loss: foreign currency translation adjustment									(2.3)	(9.3)	0.0
COMPREHENSIVE INCOME									$ 253.0	$ 74.6	$ 490.3
PER SHARE DATA
Basic earnings per share	$ (0.47)	$ 0.79	$ 1.73	$ 0.82	$ 0.02	$ (0.57)	$ 0.10	$ 1.30	$ 2.89	$ 0.91	$ 5.03
Diluted earnings per share	$ (0.47)	$ 0.77	$ 1.70	$ 0.81	$ 0.02	$ (0.57)	$ 0.10	$ 1.27	$ 2.84	$ 0.89	$ 4.92
Weighted average common shares outstanding	87,036,339	87,102,290	88,742,484	90,254,512	90,934,107	90,882,511	92,441,730	95,935,551	88,275,810	92,530,208	97,538,319
Weighted average common shares and common share equivalents outstanding	87,036,339	88,603,101	90,040,509	91,559,225	92,422,422	90,882,511	93,984,226	97,577,029	89,800,894	94,174,460	99,591,773
Dividends paid per share									$ 1.04	$ 1.23	$ 0.784