Reserves for Losses and Loss Adjustment Expenses - Summary of Information about Incurred and Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance (Detail) $ in Millions	Dec. 31, 2018 USD ($) claim	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)	Dec. 31, 2013 USD ($)	Dec. 31, 2012 USD ($)	Dec. 31, 2011 USD ($)	Dec. 31, 2010 USD ($)
Claims Development [Line Items]
Total outstanding liabilities for unpaid losses and ALAE, net of reinsurance	$ 2,516.2
U.S. Operations [Member] | Excess and Surplus Lines [Member] | Liability [Member]
Claims Development [Line Items]
Incurred Losses & ALAE, Net of Reinsurance	1,791.0
Cumulative Paid Losses & ALAE, Net of Reinsurance	952.5
Outstanding liabilities for unpaid losses and ALAE prior to 2011, net of reinsurance	54.7
Total outstanding liabilities for unpaid losses and ALAE, net of reinsurance	893.2
U.S. Operations [Member] | Excess and Surplus Lines [Member] | Liability [Member] | Accident Year 2011 [Member]
Claims Development [Line Items]
Incurred Losses & ALAE, Net of Reinsurance	187.8	$ 189.0	$ 192.8	$ 193.5	$ 200.0	$ 205.8	$ 206.0	$ 202.9
Cumulative Paid Losses & ALAE, Net of Reinsurance	174.3	170.0	162.4	146.6	122.9	91.0	53.8	17.6
IBNR & Expected Development on Reported Claims	$ 10.0
Cumulative Number of Reported Claims | claim	8,454
U.S. Operations [Member] | Excess and Surplus Lines [Member] | Liability [Member] | Accident Year 2012 [Member]
Claims Development [Line Items]
Incurred Losses & ALAE, Net of Reinsurance	$ 182.3	182.1	184.4	183.6	189.7	196.0	189.6
Cumulative Paid Losses & ALAE, Net of Reinsurance	163.4	157.5	142.4	120.8	89.1	52.8	17.2
IBNR & Expected Development on Reported Claims	$ 11.0
Cumulative Number of Reported Claims | claim	7,389
U.S. Operations [Member] | Excess and Surplus Lines [Member] | Liability [Member] | Accident Year 2013 [Member]
Claims Development [Line Items]
Incurred Losses & ALAE, Net of Reinsurance	$ 216.0	220.4	227.2	224.3	222.6	217.9
Cumulative Paid Losses & ALAE, Net of Reinsurance	179.6	163.7	135.2	100.4	60.2	17.6
IBNR & Expected Development on Reported Claims	$ 21.0
Cumulative Number of Reported Claims | claim	7,297
U.S. Operations [Member] | Excess and Surplus Lines [Member] | Liability [Member] | Accident Year 2014 [Member]
Claims Development [Line Items]
Incurred Losses & ALAE, Net of Reinsurance	$ 212.3	211.9	213.2	215.2	213.0
Cumulative Paid Losses & ALAE, Net of Reinsurance	154.5	131.6	95.9	52.2	15.0
IBNR & Expected Development on Reported Claims	$ 36.2
Cumulative Number of Reported Claims | claim	6,502
U.S. Operations [Member] | Excess and Surplus Lines [Member] | Liability [Member] | Accident Year 2015 [Member]
Claims Development [Line Items]
Incurred Losses & ALAE, Net of Reinsurance	$ 226.4	228.6	237.1	232.5
Cumulative Paid Losses & ALAE, Net of Reinsurance	131.5	91.4	51.9	16.5
IBNR & Expected Development on Reported Claims	$ 60.3
Cumulative Number of Reported Claims | claim	6,000
U.S. Operations [Member] | Excess and Surplus Lines [Member] | Liability [Member] | Accident Year 2016 [Member]
Claims Development [Line Items]
Incurred Losses & ALAE, Net of Reinsurance	$ 243.1	250.6	246.4
Cumulative Paid Losses & ALAE, Net of Reinsurance	95.5	52.8	17.4
IBNR & Expected Development on Reported Claims	$ 100.3
Cumulative Number of Reported Claims | claim	5,548
U.S. Operations [Member] | Excess and Surplus Lines [Member] | Liability [Member] | Accident Year 2017 [Member]
Claims Development [Line Items]
Incurred Losses & ALAE, Net of Reinsurance	$ 244.3	253.3
Cumulative Paid Losses & ALAE, Net of Reinsurance	38.7	11.5
IBNR & Expected Development on Reported Claims	$ 153.3
Cumulative Number of Reported Claims | claim	5,302
U.S. Operations [Member] | Excess and Surplus Lines [Member] | Liability [Member] | Accident Year 2018 [Member]
Claims Development [Line Items]
Incurred Losses & ALAE, Net of Reinsurance	$ 278.8
Cumulative Paid Losses & ALAE, Net of Reinsurance	15.0
IBNR & Expected Development on Reported Claims	$ 227.9
Cumulative Number of Reported Claims | claim	4,026
U.S. Operations [Member] | Specialty Admitted [Member] | Liability [Member]
Claims Development [Line Items]
Incurred Losses & ALAE, Net of Reinsurance	$ 1,021.2
Cumulative Paid Losses & ALAE, Net of Reinsurance	647.3
Outstanding liabilities for unpaid losses and ALAE prior to 2011, net of reinsurance	48.7
Total outstanding liabilities for unpaid losses and ALAE, net of reinsurance	422.6
U.S. Operations [Member] | Specialty Admitted [Member] | Liability [Member] | Accident Year 2011 [Member]
Claims Development [Line Items]
Incurred Losses & ALAE, Net of Reinsurance	154.0	153.7	154.0	158.2	157.5	159.0	155.1	140.3
Cumulative Paid Losses & ALAE, Net of Reinsurance	143.1	139.8	135.1	126.1	111.3	85.9	57.5	23.2
IBNR & Expected Development on Reported Claims	$ 6.3
Cumulative Number of Reported Claims | claim	28,160
U.S. Operations [Member] | Specialty Admitted [Member] | Liability [Member] | Accident Year 2012 [Member]
Claims Development [Line Items]
Incurred Losses & ALAE, Net of Reinsurance	$ 146.7	147.7	151.5	153.3	149.7	146.3	140.3
Cumulative Paid Losses & ALAE, Net of Reinsurance	131.8	127.9	120.8	105.8	80.7	51.0	20.1
IBNR & Expected Development on Reported Claims	$ 8.0
Cumulative Number of Reported Claims | claim	23,627
U.S. Operations [Member] | Specialty Admitted [Member] | Liability [Member] | Accident Year 2013 [Member]
Claims Development [Line Items]
Incurred Losses & ALAE, Net of Reinsurance	$ 130.6	131.1	133.2	136.7	133.2	126.6
Cumulative Paid Losses & ALAE, Net of Reinsurance	109.7	102.8	93.6	74.0	49.4	18.9
IBNR & Expected Development on Reported Claims	$ 10.3
Cumulative Number of Reported Claims | claim	18,948
U.S. Operations [Member] | Specialty Admitted [Member] | Liability [Member] | Accident Year 2014 [Member]
Claims Development [Line Items]
Incurred Losses & ALAE, Net of Reinsurance	$ 111.5	114.5	116.9	121.9	115.6
Cumulative Paid Losses & ALAE, Net of Reinsurance	86.1	75.3	58.7	38.8	17.4
IBNR & Expected Development on Reported Claims	$ 13.9
Cumulative Number of Reported Claims | claim	16,316
U.S. Operations [Member] | Specialty Admitted [Member] | Liability [Member] | Accident Year 2015 [Member]
Claims Development [Line Items]
Incurred Losses & ALAE, Net of Reinsurance	$ 102.3	101.7	106.7	107.3
Cumulative Paid Losses & ALAE, Net of Reinsurance	64.2	48.8	35.0	17.2
IBNR & Expected Development on Reported Claims	$ 21.1
Cumulative Number of Reported Claims | claim	14,530
U.S. Operations [Member] | Specialty Admitted [Member] | Liability [Member] | Accident Year 2016 [Member]
Claims Development [Line Items]
Incurred Losses & ALAE, Net of Reinsurance	$ 99.3	99.9	96.1
Cumulative Paid Losses & ALAE, Net of Reinsurance	48.6	31.7	11.1
IBNR & Expected Development on Reported Claims	$ 22.1
Cumulative Number of Reported Claims | claim	11,583
U.S. Operations [Member] | Specialty Admitted [Member] | Liability [Member] | Accident Year 2017 [Member]
Claims Development [Line Items]
Incurred Losses & ALAE, Net of Reinsurance	$ 129.5	121.5
Cumulative Paid Losses & ALAE, Net of Reinsurance	44.4	16.3
IBNR & Expected Development on Reported Claims	$ 50.2
Cumulative Number of Reported Claims | claim	12,897
U.S. Operations [Member] | Specialty Admitted [Member] | Liability [Member] | Accident Year 2018 [Member]
Claims Development [Line Items]
Incurred Losses & ALAE, Net of Reinsurance	$ 147.3
Cumulative Paid Losses & ALAE, Net of Reinsurance	19.4
IBNR & Expected Development on Reported Claims	$ 82.8
Cumulative Number of Reported Claims | claim	13,351
U.S. Operations [Member] | Specialty Admitted [Member] | Professional [Member]
Claims Development [Line Items]
Incurred Losses & ALAE, Net of Reinsurance	$ 318.3
Cumulative Paid Losses & ALAE, Net of Reinsurance	167.1
Outstanding liabilities for unpaid losses and ALAE prior to 2011, net of reinsurance	0.1
Total outstanding liabilities for unpaid losses and ALAE, net of reinsurance	151.3
U.S. Operations [Member] | Specialty Admitted [Member] | Professional [Member] | Accident Year 2011 [Member]
Claims Development [Line Items]
Incurred Losses & ALAE, Net of Reinsurance	26.0	26.6	26.9	28.2	32.5	35.0	35.0	35.0
Cumulative Paid Losses & ALAE, Net of Reinsurance	25.8	25.7	25.4	24.0	22.0	17.8	11.8	3.2
IBNR & Expected Development on Reported Claims	$ 0.3
Cumulative Number of Reported Claims | claim	820
U.S. Operations [Member] | Specialty Admitted [Member] | Professional [Member] | Accident Year 2012 [Member]
Claims Development [Line Items]
Incurred Losses & ALAE, Net of Reinsurance	$ 24.9	24.5	24.0	25.8	28.6	28.3	27.8
Cumulative Paid Losses & ALAE, Net of Reinsurance	23.5	22.6	21.4	19.9	16.9	8.6	2.3
IBNR & Expected Development on Reported Claims	$ 0.4
Cumulative Number of Reported Claims | claim	640
U.S. Operations [Member] | Specialty Admitted [Member] | Professional [Member] | Accident Year 2013 [Member]
Claims Development [Line Items]
Incurred Losses & ALAE, Net of Reinsurance	$ 19.5	19.8	19.0	21.1	21.5	20.9
Cumulative Paid Losses & ALAE, Net of Reinsurance	17.5	17.6	14.2	10.9	6.3	1.9
IBNR & Expected Development on Reported Claims	$ 1.2
Cumulative Number of Reported Claims | claim	620
U.S. Operations [Member] | Specialty Admitted [Member] | Professional [Member] | Accident Year 2014 [Member]
Claims Development [Line Items]
Incurred Losses & ALAE, Net of Reinsurance	$ 36.2	33.7	26.0	22.4	22.4
Cumulative Paid Losses & ALAE, Net of Reinsurance	25.5	24.1	15.1	5.4	2.3
IBNR & Expected Development on Reported Claims	$ 2.8
Cumulative Number of Reported Claims | claim	1,043
U.S. Operations [Member] | Specialty Admitted [Member] | Professional [Member] | Accident Year 2015 [Member]
Claims Development [Line Items]
Incurred Losses & ALAE, Net of Reinsurance	$ 34.0	33.2	29.5	29.9
Cumulative Paid Losses & ALAE, Net of Reinsurance	20.8	15.6	8.3	1.8
IBNR & Expected Development on Reported Claims	$ 2.1
Cumulative Number of Reported Claims | claim	1,816
U.S. Operations [Member] | Specialty Admitted [Member] | Professional [Member] | Accident Year 2016 [Member]
Claims Development [Line Items]
Incurred Losses & ALAE, Net of Reinsurance	$ 45.1	44.8	44.2
Cumulative Paid Losses & ALAE, Net of Reinsurance	24.6	11.9	2.4
IBNR & Expected Development on Reported Claims	$ 13.1
Cumulative Number of Reported Claims | claim	3,226
U.S. Operations [Member] | Specialty Admitted [Member] | Professional [Member] | Accident Year 2017 [Member]
Claims Development [Line Items]
Incurred Losses & ALAE, Net of Reinsurance	$ 61.8	60.1
Cumulative Paid Losses & ALAE, Net of Reinsurance	24.9	3.5
IBNR & Expected Development on Reported Claims	$ 15.9
Cumulative Number of Reported Claims | claim	3,669
U.S. Operations [Member] | Specialty Admitted [Member] | Professional [Member] | Accident Year 2018 [Member]
Claims Development [Line Items]
Incurred Losses & ALAE, Net of Reinsurance	$ 70.8
Cumulative Paid Losses & ALAE, Net of Reinsurance	4.5
IBNR & Expected Development on Reported Claims	$ 55.0
Cumulative Number of Reported Claims | claim	3,603
U.S. Operations [Member] | Specialty Admitted [Member] | Specialty [Member]
Claims Development [Line Items]
Incurred Losses & ALAE, Net of Reinsurance	$ 64.6
Cumulative Paid Losses & ALAE, Net of Reinsurance	12.6
Outstanding liabilities for unpaid losses and ALAE prior to 2011, net of reinsurance	0.0
Total outstanding liabilities for unpaid losses and ALAE, net of reinsurance	52.0
U.S. Operations [Member] | Specialty Admitted [Member] | Specialty [Member] | Accident Year 2011 [Member]
Claims Development [Line Items]
Incurred Losses & ALAE, Net of Reinsurance	1.7	2.0	2.6	3.6	3.4	3.4	3.9	0.2
Cumulative Paid Losses & ALAE, Net of Reinsurance	1.7	1.7	1.7	1.2	1.3	1.4	1.6	0.0
IBNR & Expected Development on Reported Claims	$ 0.0
Cumulative Number of Reported Claims | claim	80
U.S. Operations [Member] | Specialty Admitted [Member] | Specialty [Member] | Accident Year 2012 [Member]
Claims Development [Line Items]
Incurred Losses & ALAE, Net of Reinsurance	$ 3.5	3.9	4.0	4.3	4.9	6.7	7.5
Cumulative Paid Losses & ALAE, Net of Reinsurance	3.3	3.4	3.3	3.3	3.3	3.3	3.6
IBNR & Expected Development on Reported Claims	$ 0.2
Cumulative Number of Reported Claims | claim	129
U.S. Operations [Member] | Specialty Admitted [Member] | Specialty [Member] | Accident Year 2013 [Member]
Claims Development [Line Items]
Incurred Losses & ALAE, Net of Reinsurance	$ 0.9	1.7	2.5	4.6	8.6	10.0
Cumulative Paid Losses & ALAE, Net of Reinsurance	0.9	0.9	0.9	0.9	0.9	0.4
IBNR & Expected Development on Reported Claims	$ 0.0
Cumulative Number of Reported Claims | claim	50
U.S. Operations [Member] | Specialty Admitted [Member] | Specialty [Member] | Accident Year 2014 [Member]
Claims Development [Line Items]
Incurred Losses & ALAE, Net of Reinsurance	$ 4.8	6.0	8.9	13.1	13.1
Cumulative Paid Losses & ALAE, Net of Reinsurance	4.1	4.0	4.0	3.3	1.1
IBNR & Expected Development on Reported Claims	$ 0.8
Cumulative Number of Reported Claims | claim	50
U.S. Operations [Member] | Specialty Admitted [Member] | Specialty [Member] | Accident Year 2015 [Member]
Claims Development [Line Items]
Incurred Losses & ALAE, Net of Reinsurance	$ 5.5	9.5	14.3	14.8
Cumulative Paid Losses & ALAE, Net of Reinsurance	0.3	0.2	0.1	0.2
IBNR & Expected Development on Reported Claims	$ 2.9
Cumulative Number of Reported Claims | claim	24
U.S. Operations [Member] | Specialty Admitted [Member] | Specialty [Member] | Accident Year 2016 [Member]
Claims Development [Line Items]
Incurred Losses & ALAE, Net of Reinsurance	$ 11.2	15.0	15.0
Cumulative Paid Losses & ALAE, Net of Reinsurance	2.2	1.6	1.3
IBNR & Expected Development on Reported Claims	$ 5.5
Cumulative Number of Reported Claims | claim	58
U.S. Operations [Member] | Specialty Admitted [Member] | Specialty [Member] | Accident Year 2017 [Member]
Claims Development [Line Items]
Incurred Losses & ALAE, Net of Reinsurance	$ 16.2	16.2
Cumulative Paid Losses & ALAE, Net of Reinsurance	0.1	0.3
IBNR & Expected Development on Reported Claims	$ 16.2
Cumulative Number of Reported Claims | claim	84
U.S. Operations [Member] | Specialty Admitted [Member] | Specialty [Member] | Accident Year 2018 [Member]
Claims Development [Line Items]
Incurred Losses & ALAE, Net of Reinsurance	$ 20.8
Cumulative Paid Losses & ALAE, Net of Reinsurance	0.0
IBNR & Expected Development on Reported Claims	$ 19.8
Cumulative Number of Reported Claims | claim	71
International Operations [Member] | Reinsurance Division [Member] | Property [Member]
Claims Development [Line Items]
Incurred Losses & ALAE, Net of Reinsurance	$ 698.1
Cumulative Paid Losses & ALAE, Net of Reinsurance	604.5
Outstanding liabilities for unpaid losses and ALAE prior to 2011, net of reinsurance	3.6
Total outstanding liabilities for unpaid losses and ALAE, net of reinsurance	97.2
International Operations [Member] | Reinsurance Division [Member] | Property [Member] | Accident Year 2011 [Member]
Claims Development [Line Items]
Incurred Losses & ALAE, Net of Reinsurance	138.9	132.3	104.3	104.4	105.1	107.1	105.0	116.3
Cumulative Paid Losses & ALAE, Net of Reinsurance	134.1	127.2	99.0	97.8	95.5	87.7	66.5	40.9
IBNR & Expected Development on Reported Claims	$ 0.3
Cumulative Number of Reported Claims | claim	459
International Operations [Member] | Reinsurance Division [Member] | Property [Member] | Accident Year 2012 [Member]
Claims Development [Line Items]
Incurred Losses & ALAE, Net of Reinsurance	$ 66.9	62.7	46.4	51.6	50.3	51.3	47.2
Cumulative Paid Losses & ALAE, Net of Reinsurance	64.6	58.7	44.0	49.6	40.6	31.1	12.4
IBNR & Expected Development on Reported Claims	$ 0.2
Cumulative Number of Reported Claims | claim	277
International Operations [Member] | Reinsurance Division [Member] | Property [Member] | Accident Year 2013 [Member]
Claims Development [Line Items]
Incurred Losses & ALAE, Net of Reinsurance	$ 32.4	33.0	31.4	33.1	33.6	32.1
Cumulative Paid Losses & ALAE, Net of Reinsurance	31.1	31.0	29.0	26.5	16.7	4.2
IBNR & Expected Development on Reported Claims	$ 0.3
Cumulative Number of Reported Claims | claim	219
International Operations [Member] | Reinsurance Division [Member] | Property [Member] | Accident Year 2014 [Member]
Claims Development [Line Items]
Incurred Losses & ALAE, Net of Reinsurance	$ 38.1	35.3	24.2	26.5	26.5
Cumulative Paid Losses & ALAE, Net of Reinsurance	36.9	36.4	18.3	12.7	2.8
IBNR & Expected Development on Reported Claims	$ 0.5
Cumulative Number of Reported Claims | claim	221
International Operations [Member] | Reinsurance Division [Member] | Property [Member] | Accident Year 2015 [Member]
Claims Development [Line Items]
Incurred Losses & ALAE, Net of Reinsurance	$ 27.1	32.6	23.3	27.1
Cumulative Paid Losses & ALAE, Net of Reinsurance	20.6	20.3	11.0	4.3
IBNR & Expected Development on Reported Claims	$ 3.2
Cumulative Number of Reported Claims | claim	219
International Operations [Member] | Reinsurance Division [Member] | Property [Member] | Accident Year 2016 [Member]
Claims Development [Line Items]
Incurred Losses & ALAE, Net of Reinsurance	$ 169.1	175.5	43.7
Cumulative Paid Losses & ALAE, Net of Reinsurance	153.0	142.3	13.6
IBNR & Expected Development on Reported Claims	$ 11.0
Cumulative Number of Reported Claims | claim	381
International Operations [Member] | Reinsurance Division [Member] | Property [Member] | Accident Year 2017 [Member]
Claims Development [Line Items]
Incurred Losses & ALAE, Net of Reinsurance	$ 158.0	157.8
Cumulative Paid Losses & ALAE, Net of Reinsurance	139.5	84.2
IBNR & Expected Development on Reported Claims	$ (28.8)
Cumulative Number of Reported Claims | claim	774
International Operations [Member] | Reinsurance Division [Member] | Property [Member] | Accident Year 2018 [Member]
Claims Development [Line Items]
Incurred Losses & ALAE, Net of Reinsurance	$ 67.6
Cumulative Paid Losses & ALAE, Net of Reinsurance	24.7
IBNR & Expected Development on Reported Claims	$ 4.2
Cumulative Number of Reported Claims | claim	391
International Operations [Member] | Bermuda Insurance [Member] | Liability [Member]
Claims Development [Line Items]
Incurred Losses & ALAE, Net of Reinsurance	$ 79.3
Cumulative Paid Losses & ALAE, Net of Reinsurance	27.1
Outstanding liabilities for unpaid losses and ALAE prior to 2011, net of reinsurance	0.0
Total outstanding liabilities for unpaid losses and ALAE, net of reinsurance	52.2
International Operations [Member] | Bermuda Insurance [Member] | Liability [Member] | Accident Year 2011 [Member]
Claims Development [Line Items]
Incurred Losses & ALAE, Net of Reinsurance	0.0	1.0	1.6	2.2	4.4	6.6	6.6	6.6
Cumulative Paid Losses & ALAE, Net of Reinsurance	0.0	0.0	0.0	0.0	0.0	0.0	0.0	0.0
IBNR & Expected Development on Reported Claims	$ 0.0
Cumulative Number of Reported Claims | claim	1,423
International Operations [Member] | Bermuda Insurance [Member] | Liability [Member] | Accident Year 2012 [Member]
Claims Development [Line Items]
Incurred Losses & ALAE, Net of Reinsurance	$ 0.0	1.7	4.4	5.6	7.4	7.4	7.4
Cumulative Paid Losses & ALAE, Net of Reinsurance	0.0	0.0	0.0	0.0	0.0	0.0	0.0
IBNR & Expected Development on Reported Claims	$ 0.0
Cumulative Number of Reported Claims | claim	1,381
International Operations [Member] | Bermuda Insurance [Member] | Liability [Member] | Accident Year 2013 [Member]
Claims Development [Line Items]
Incurred Losses & ALAE, Net of Reinsurance	$ 2.2	4.9	8.5	8.5	8.5	8.5
Cumulative Paid Losses & ALAE, Net of Reinsurance	2.3	2.3	0.0	0.0	0.0	0.0
IBNR & Expected Development on Reported Claims	$ 0.0
Cumulative Number of Reported Claims | claim	1,190
International Operations [Member] | Bermuda Insurance [Member] | Liability [Member] | Accident Year 2014 [Member]
Claims Development [Line Items]
Incurred Losses & ALAE, Net of Reinsurance	$ 1.5	6.2	9.8	9.8	9.8
Cumulative Paid Losses & ALAE, Net of Reinsurance	1.2	0.1	0.1	0.0	0.0
IBNR & Expected Development on Reported Claims	$ 0.2
Cumulative Number of Reported Claims | claim	1,329
International Operations [Member] | Bermuda Insurance [Member] | Liability [Member] | Accident Year 2015 [Member]
Claims Development [Line Items]
Incurred Losses & ALAE, Net of Reinsurance	$ 35.4	24.8	14.3	11.3
Cumulative Paid Losses & ALAE, Net of Reinsurance	20.3	16.1	0.0	0.0
IBNR & Expected Development on Reported Claims	$ 1.0
Cumulative Number of Reported Claims | claim	1,567
International Operations [Member] | Bermuda Insurance [Member] | Liability [Member] | Accident Year 2016 [Member]
Claims Development [Line Items]
Incurred Losses & ALAE, Net of Reinsurance	$ 14.0	14.0	13.9
Cumulative Paid Losses & ALAE, Net of Reinsurance	0.0	0.0	0.0
IBNR & Expected Development on Reported Claims	$ 13.9
Cumulative Number of Reported Claims | claim	1,874
International Operations [Member] | Bermuda Insurance [Member] | Liability [Member] | Accident Year 2017 [Member]
Claims Development [Line Items]
Incurred Losses & ALAE, Net of Reinsurance	$ 17.3	17.1
Cumulative Paid Losses & ALAE, Net of Reinsurance	3.3	0.0
IBNR & Expected Development on Reported Claims	$ 12.2
Cumulative Number of Reported Claims | claim	1,900
International Operations [Member] | Bermuda Insurance [Member] | Liability [Member] | Accident Year 2018 [Member]
Claims Development [Line Items]
Incurred Losses & ALAE, Net of Reinsurance	$ 8.9
Cumulative Paid Losses & ALAE, Net of Reinsurance	0.0
IBNR & Expected Development on Reported Claims	$ 8.8
Cumulative Number of Reported Claims | claim	839
International Operations [Member] | Syndicate 1200 [Member] | Liability [Member]
Claims Development [Line Items]
Incurred Losses & ALAE, Net of Reinsurance	$ 190.7
Cumulative Paid Losses & ALAE, Net of Reinsurance	96.4
Total outstanding liabilities for unpaid losses and ALAE, net of reinsurance	94.3
International Operations [Member] | Syndicate 1200 [Member] | Liability [Member] | Accident Year 2010 [Member]
Claims Development [Line Items]
Incurred Losses & ALAE, Net of Reinsurance	5.7	5.8	5.7	5.8	5.9	6.1	4.4	4.8	$ 5.7
Cumulative Paid Losses & ALAE, Net of Reinsurance	4.3	4.0	3.7	2.8	2.0	1.1	0.6	0.2	0.0
IBNR & Expected Development on Reported Claims	0.1
International Operations [Member] | Syndicate 1200 [Member] | Liability [Member] | Accident Year 2011 [Member]
Claims Development [Line Items]
Incurred Losses & ALAE, Net of Reinsurance	10.7	10.6	10.1	10.2	10.9	10.8	8.6	8.0
Cumulative Paid Losses & ALAE, Net of Reinsurance	8.7	7.8	6.9	5.4	3.4	1.6	0.8	0.2
IBNR & Expected Development on Reported Claims	0.2
International Operations [Member] | Syndicate 1200 [Member] | Liability [Member] | Accident Year 2012 [Member]
Claims Development [Line Items]
Incurred Losses & ALAE, Net of Reinsurance	15.0	14.5	13.8	14.2	14.9	10.8	8.7
Cumulative Paid Losses & ALAE, Net of Reinsurance	11.8	10.1	8.2	5.8	2.6	1.1	0.4
IBNR & Expected Development on Reported Claims	0.6
International Operations [Member] | Syndicate 1200 [Member] | Liability [Member] | Accident Year 2013 [Member]
Claims Development [Line Items]
Incurred Losses & ALAE, Net of Reinsurance	25.1	24.3	24.4	26.2	26.7	22.7
Cumulative Paid Losses & ALAE, Net of Reinsurance	19.7	15.6	11.3	7.0	3.2	1.5
IBNR & Expected Development on Reported Claims	2.5
International Operations [Member] | Syndicate 1200 [Member] | Liability [Member] | Accident Year 2014 [Member]
Claims Development [Line Items]
Incurred Losses & ALAE, Net of Reinsurance	33.8	32.5	33.6	35.9	37.1
Cumulative Paid Losses & ALAE, Net of Reinsurance	20.2	13.6	9.8	4.5	1.9
IBNR & Expected Development on Reported Claims	5.0
International Operations [Member] | Syndicate 1200 [Member] | Liability [Member] | Accident Year 2015 [Member]
Claims Development [Line Items]
Incurred Losses & ALAE, Net of Reinsurance	29.6	29.2	29.6	34.2
Cumulative Paid Losses & ALAE, Net of Reinsurance	12.3	7.4	5.1	0.8
IBNR & Expected Development on Reported Claims	8.2
International Operations [Member] | Syndicate 1200 [Member] | Liability [Member] | Accident Year 2016 [Member]
Claims Development [Line Items]
Incurred Losses & ALAE, Net of Reinsurance	26.1	26.4	25.5
Cumulative Paid Losses & ALAE, Net of Reinsurance	10.6	5.7	1.8
IBNR & Expected Development on Reported Claims	10.2
International Operations [Member] | Syndicate 1200 [Member] | Liability [Member] | Accident Year 2017 [Member]
Claims Development [Line Items]
Incurred Losses & ALAE, Net of Reinsurance	23.3	24.5
Cumulative Paid Losses & ALAE, Net of Reinsurance	6.6	1.9
IBNR & Expected Development on Reported Claims	15.2
International Operations [Member] | Syndicate 1200 [Member] | Liability [Member] | Accident Year 2018 [Member]
Claims Development [Line Items]
Incurred Losses & ALAE, Net of Reinsurance	21.4
Cumulative Paid Losses & ALAE, Net of Reinsurance	2.2
IBNR & Expected Development on Reported Claims	17.6
International Operations [Member] | Syndicate 1200 [Member] | Professional [Member]
Claims Development [Line Items]
Incurred Losses & ALAE, Net of Reinsurance	213.1
Cumulative Paid Losses & ALAE, Net of Reinsurance	113.5
Total outstanding liabilities for unpaid losses and ALAE, net of reinsurance	99.6
International Operations [Member] | Syndicate 1200 [Member] | Professional [Member] | Accident Year 2010 [Member]
Claims Development [Line Items]
Incurred Losses & ALAE, Net of Reinsurance	9.7	9.5	9.5	9.4	10.1	11.2	12.7	12.8	15.5
Cumulative Paid Losses & ALAE, Net of Reinsurance	7.6	6.8	6.5	5.1	3.8	3.0	2.0	0.9	0.1
IBNR & Expected Development on Reported Claims	0.1
International Operations [Member] | Syndicate 1200 [Member] | Professional [Member] | Accident Year 2011 [Member]
Claims Development [Line Items]
Incurred Losses & ALAE, Net of Reinsurance	15.7	15.1	14.7	14.7	15.5	18.5	21.0	19.1
Cumulative Paid Losses & ALAE, Net of Reinsurance	12.8	11.3	10.6	8.3	6.6	4.1	2.5	1.0
IBNR & Expected Development on Reported Claims	0.4
International Operations [Member] | Syndicate 1200 [Member] | Professional [Member] | Accident Year 2012 [Member]
Claims Development [Line Items]
Incurred Losses & ALAE, Net of Reinsurance	15.4	14.9	14.0	14.0	14.1	13.8	13.9
Cumulative Paid Losses & ALAE, Net of Reinsurance	11.2	9.5	8.1	5.7	4.3	1.8	0.6
IBNR & Expected Development on Reported Claims	0.8
International Operations [Member] | Syndicate 1200 [Member] | Professional [Member] | Accident Year 2013 [Member]
Claims Development [Line Items]
Incurred Losses & ALAE, Net of Reinsurance	23.1	22.3	22.2	22.6	22.4	22.5
Cumulative Paid Losses & ALAE, Net of Reinsurance	17.6	15.4	11.6	7.0	3.7	1.7
IBNR & Expected Development on Reported Claims	3.0
International Operations [Member] | Syndicate 1200 [Member] | Professional [Member] | Accident Year 2014 [Member]
Claims Development [Line Items]
Incurred Losses & ALAE, Net of Reinsurance	40.9	39.0	35.9	35.7	34.5
Cumulative Paid Losses & ALAE, Net of Reinsurance	27.9	23.7	14.6	6.3	1.6
IBNR & Expected Development on Reported Claims	6.5
International Operations [Member] | Syndicate 1200 [Member] | Professional [Member] | Accident Year 2015 [Member]
Claims Development [Line Items]
Incurred Losses & ALAE, Net of Reinsurance	38.7	38.5	37.3	37.8
Cumulative Paid Losses & ALAE, Net of Reinsurance	19.7	14.8	8.2	2.2
IBNR & Expected Development on Reported Claims	11.3
International Operations [Member] | Syndicate 1200 [Member] | Professional [Member] | Accident Year 2016 [Member]
Claims Development [Line Items]
Incurred Losses & ALAE, Net of Reinsurance	26.4	26.9	32.8
Cumulative Paid Losses & ALAE, Net of Reinsurance	10.6	5.8	2.0
IBNR & Expected Development on Reported Claims	10.8
International Operations [Member] | Syndicate 1200 [Member] | Professional [Member] | Accident Year 2017 [Member]
Claims Development [Line Items]
Incurred Losses & ALAE, Net of Reinsurance	22.0	24.5
Cumulative Paid Losses & ALAE, Net of Reinsurance	5.1	1.2
IBNR & Expected Development on Reported Claims	14.6
International Operations [Member] | Syndicate 1200 [Member] | Professional [Member] | Accident Year 2018 [Member]
Claims Development [Line Items]
Incurred Losses & ALAE, Net of Reinsurance	21.2
Cumulative Paid Losses & ALAE, Net of Reinsurance	1.0
IBNR & Expected Development on Reported Claims	17.6
International Operations [Member] | Syndicate 1200 [Member] | Specialty [Member]
Claims Development [Line Items]
Incurred Losses & ALAE, Net of Reinsurance	609.5
Cumulative Paid Losses & ALAE, Net of Reinsurance	462.7
Total outstanding liabilities for unpaid losses and ALAE, net of reinsurance	146.8
International Operations [Member] | Syndicate 1200 [Member] | Specialty [Member] | Accident Year 2010 [Member]
Claims Development [Line Items]
Incurred Losses & ALAE, Net of Reinsurance	12.0	12.0	12.0	12.1	12.1	13.0	13.6	15.7	12.8
Cumulative Paid Losses & ALAE, Net of Reinsurance	10.2	10.0	10.0	9.7	9.0	9.2	8.8	8.6	1.8
IBNR & Expected Development on Reported Claims	0.0
International Operations [Member] | Syndicate 1200 [Member] | Specialty [Member] | Accident Year 2011 [Member]
Claims Development [Line Items]
Incurred Losses & ALAE, Net of Reinsurance	32.7	33.1	33.2	33.2	34.0	38.7	40.0	38.3
Cumulative Paid Losses & ALAE, Net of Reinsurance	29.4	29.5	29.1	28.5	27.1	23.6	19.5	11.5
IBNR & Expected Development on Reported Claims	0.0
International Operations [Member] | Syndicate 1200 [Member] | Specialty [Member] | Accident Year 2012 [Member]
Claims Development [Line Items]
Incurred Losses & ALAE, Net of Reinsurance	58.2	58.8	59.0	59.2	60.7	56.4	52.5
Cumulative Paid Losses & ALAE, Net of Reinsurance	50.1	49.8	48.7	45.7	38.8	27.4	17.9
IBNR & Expected Development on Reported Claims	0.0
International Operations [Member] | Syndicate 1200 [Member] | Specialty [Member] | Accident Year 2013 [Member]
Claims Development [Line Items]
Incurred Losses & ALAE, Net of Reinsurance	81.5	81.8	82.1	82.5	81.2	75.7
Cumulative Paid Losses & ALAE, Net of Reinsurance	78.6	78.2	76.2	68.7	52.3	30.9
IBNR & Expected Development on Reported Claims	(0.5)
International Operations [Member] | Syndicate 1200 [Member] | Specialty [Member] | Accident Year 2014 [Member]
Claims Development [Line Items]
Incurred Losses & ALAE, Net of Reinsurance	100.6	101.1	99.7	98.4	92.1
Cumulative Paid Losses & ALAE, Net of Reinsurance	90.4	88.9	82.5	71.4	37.7
IBNR & Expected Development on Reported Claims	0.7
International Operations [Member] | Syndicate 1200 [Member] | Specialty [Member] | Accident Year 2015 [Member]
Claims Development [Line Items]
Incurred Losses & ALAE, Net of Reinsurance	95.3	93.8	87.8	89.8
Cumulative Paid Losses & ALAE, Net of Reinsurance	74.0	66.2	54.0	30.9
IBNR & Expected Development on Reported Claims	3.0
International Operations [Member] | Syndicate 1200 [Member] | Specialty [Member] | Accident Year 2016 [Member]
Claims Development [Line Items]
Incurred Losses & ALAE, Net of Reinsurance	87.7	84.5	86.0
Cumulative Paid Losses & ALAE, Net of Reinsurance	68.7	59.0	37.3
IBNR & Expected Development on Reported Claims	6.6
International Operations [Member] | Syndicate 1200 [Member] | Specialty [Member] | Accident Year 2017 [Member]
Claims Development [Line Items]
Incurred Losses & ALAE, Net of Reinsurance	76.5	80.1
Cumulative Paid Losses & ALAE, Net of Reinsurance	42.6	20.1
IBNR & Expected Development on Reported Claims	21.8
International Operations [Member] | Syndicate 1200 [Member] | Specialty [Member] | Accident Year 2018 [Member]
Claims Development [Line Items]
Incurred Losses & ALAE, Net of Reinsurance	65.0
Cumulative Paid Losses & ALAE, Net of Reinsurance	18.7
IBNR & Expected Development on Reported Claims	40.3
International Operations [Member] | Syndicate 1200 [Member] | Property [Member]
Claims Development [Line Items]
Incurred Losses & ALAE, Net of Reinsurance	693.6
Cumulative Paid Losses & ALAE, Net of Reinsurance	567.6
Total outstanding liabilities for unpaid losses and ALAE, net of reinsurance	126.0
International Operations [Member] | Syndicate 1200 [Member] | Property [Member] | Accident Year 2010 [Member]
Claims Development [Line Items]
Incurred Losses & ALAE, Net of Reinsurance	51.1	51.4	50.7	51.2	51.5	54.2	54.6	58.2	50.7
Cumulative Paid Losses & ALAE, Net of Reinsurance	43.4	42.6	41.5	40.0	35.8	33.8	28.0	19.5	$ 1.8
IBNR & Expected Development on Reported Claims	0.0
International Operations [Member] | Syndicate 1200 [Member] | Property [Member] | Accident Year 2011 [Member]
Claims Development [Line Items]
Incurred Losses & ALAE, Net of Reinsurance	91.8	92.4	92.4	92.9	94.8	107.5	113.3	108.0
Cumulative Paid Losses & ALAE, Net of Reinsurance	82.4	82.7	81.0	79.3	73.5	61.8	47.1	$ 23.4
IBNR & Expected Development on Reported Claims	0.2
International Operations [Member] | Syndicate 1200 [Member] | Property [Member] | Accident Year 2012 [Member]
Claims Development [Line Items]
Incurred Losses & ALAE, Net of Reinsurance	89.9	90.8	90.9	91.9	92.8	88.5	88.7
Cumulative Paid Losses & ALAE, Net of Reinsurance	77.7	77.5	76.3	73.9	62.8	47.6	$ 29.4
IBNR & Expected Development on Reported Claims	0.2
International Operations [Member] | Syndicate 1200 [Member] | Property [Member] | Accident Year 2013 [Member]
Claims Development [Line Items]
Incurred Losses & ALAE, Net of Reinsurance	75.6	76.5	76.7	78.2	79.2	83.3
Cumulative Paid Losses & ALAE, Net of Reinsurance	73.5	74.2	73.3	69.2	56.7	$ 44.4
IBNR & Expected Development on Reported Claims	0.1
International Operations [Member] | Syndicate 1200 [Member] | Property [Member] | Accident Year 2014 [Member]
Claims Development [Line Items]
Incurred Losses & ALAE, Net of Reinsurance	65.3	66.0	65.5	64.1	69.7
Cumulative Paid Losses & ALAE, Net of Reinsurance	59.9	60.0	57.5	51.4	$ 29.4
IBNR & Expected Development on Reported Claims	0.1
International Operations [Member] | Syndicate 1200 [Member] | Property [Member] | Accident Year 2015 [Member]
Claims Development [Line Items]
Incurred Losses & ALAE, Net of Reinsurance	74.0	73.4	66.2	55.9
Cumulative Paid Losses & ALAE, Net of Reinsurance	59.1	52.6	42.7	$ 22.8
IBNR & Expected Development on Reported Claims	0.1
International Operations [Member] | Syndicate 1200 [Member] | Property [Member] | Accident Year 2016 [Member]
Claims Development [Line Items]
Incurred Losses & ALAE, Net of Reinsurance	91.8	86.6	70.7
Cumulative Paid Losses & ALAE, Net of Reinsurance	78.8	64.1	$ 38.8
IBNR & Expected Development on Reported Claims	0.8
International Operations [Member] | Syndicate 1200 [Member] | Property [Member] | Accident Year 2017 [Member]
Claims Development [Line Items]
Incurred Losses & ALAE, Net of Reinsurance	91.8	85.2
Cumulative Paid Losses & ALAE, Net of Reinsurance	62.7	$ 29.7
IBNR & Expected Development on Reported Claims	15.7
International Operations [Member] | Syndicate 1200 [Member] | Property [Member] | Accident Year 2018 [Member]
Claims Development [Line Items]
Incurred Losses & ALAE, Net of Reinsurance	62.3
Cumulative Paid Losses & ALAE, Net of Reinsurance	30.1
IBNR & Expected Development on Reported Claims	$ 31.2