Consolidated Statements Of Operations And Comprehensive Income - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
REVENUES:
Gross premiums written	$ 632.3	$ 754.1	$ 826.0	$ 880.6	$ 565.7	$ 707.9	$ 760.4	$ 901.4	$ 3,093.0	$ 2,935.4	$ 2,738.7
Premiums ceded	(167.5)	(147.1)	(222.3)	(108.1)	(137.9)	(139.2)	(206.5)	(129.8)	(645.0)	(613.4)	(618.2)
Net premiums written	464.8	607.0	603.7	772.5	427.8	568.7	553.9	771.6	2,448.0	2,322.0	2,120.5
Change in unearned premiums	158.0	43.7	42.7	(204.0)	145.7	(27.0)	(16.7)	(241.3)	40.4	(139.3)	(114.6)
Net premiums earned	622.8	650.7	646.4	568.5	573.5	541.7	537.2	530.3	2,488.4	2,182.7	2,005.9
Net investment income	49.1	45.7	42.8	44.5	49.1	43.4	36.8	47.6	182.1	176.9	157.6
Net realized investment (losses) gains	(38.8)	(113.6)	(20.2)	45.0	(15.4)	(35.1)	85.3	54.2	(127.6)	89.0	59.5
Other income	1.0	0.7	0.9	0.9	1.1	1.0	0.0	0.0	3.5	2.1	0.0
Total revenues	634.1	583.5	669.9	658.9	608.3	551.0	659.3	632.1	2,546.4	2,450.7	2,223.0
EXPENSES:
Net losses and loss expenses	412.7	416.9	431.5	325.2	273.0	336.1	314.9	275.3	1,586.3	1,199.2	1,123.2
Acquisition costs	96.0	100.1	100.6	78.7	80.7	72.4	74.3	67.7	375.4	295.1	252.7
General and administrative expenses	95.0	105.8	108.4	97.1	100.9	88.3	96.2	80.3	406.3	365.7	352.3
Other expense	1.9	1.3	1.2	1.8	2.0	6.6	0.0	0.0	6.2	8.6	0.0
Amortization and impairment of intangible assets	3.7	2.7	2.8	0.6	0.6	0.6	0.7	0.6	9.8	2.5	2.5
Interest expense	18.1	14.5	14.5	14.3	14.3	14.3	14.6	14.5	61.4	57.8	56.5
Foreign exchange loss	0.9	(0.8)	1.3	9.9	0.0	0.3	0.7	0.0	11.3	1.0	8.0
Total expenses	628.3	640.5	660.3	527.6	471.5	518.6	501.4	438.4	2,456.7	1,929.9	1,795.2
Income before income taxes	5.8	(57.0)	9.6	131.3	136.8	32.4	157.9	193.7	89.7	520.8	427.8
Income tax expense	4.0	(5.3)	0.1	7.0	6.2	1.5	6.2	16.6	5.8	30.5	9.8
NET INCOME	$ 1.8	$ (51.7)	$ 9.5	$ 124.3	$ 130.6	$ 30.9	$ 151.7	$ 177.1	83.9	490.3	418.0
Other comprehensive loss:
Other comprehensive loss: foreign currency translation adjustment, net of tax									(9.3)	0.0	0.0
COMPREHENSIVE INCOME									$ 74.6	$ 490.3	$ 418.0
PER SHARE DATA
Basic earnings per share	$ 0.02	$ (0.57)	$ 0.10	$ 1.30	$ 1.35	$ 0.32	$ 1.55	$ 1.78	$ 0.91	$ 5.03	$ 4.08
Diluted earnings per share	$ 0.02	$ (0.57)	$ 0.10	$ 1.27	$ 1.33	$ 0.31	$ 1.52	$ 1.74	$ 0.89	$ 4.92	$ 3.98
Weighted average common shares outstanding	90,934,107	90,882,511	92,441,730	95,935,551	96,386,796	96,458,231	97,809,639	99,545,187	92,530,208	97,538,319	102,464,715
Weighted average common shares and common share equivalents outstanding	92,422,422	90,882,511	93,984,226	97,577,029	98,394,432	98,444,238	99,724,802	101,584,662	94,174,460	99,591,773	104,865,834
Dividends paid per share									$ 1.23	$ 0.784	$ 0.458