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Reserves for Losses and Adjustment Expenses Short-duration Insurance Contracts, Claims by Accident Year (Details) $ in Millions	Dec. 31, 2017 USD ($) Integer	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)	Dec. 31, 2013 USD ($)	Dec. 31, 2012 USD ($)
Claims Development [Line Items]
Liability for claims and claim adjustment expenses, net of reinsurance	$ 5,183.7
Property Insurance
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expense, Net	1,154.6
Cumulative paid claims and allocated claim adjustment expense, Net	824.5
All outstanding liabilities for 2012 and subsequent years, net of reinsurance	330.1
All outstanding liabilities before 2012, net of reinsurance	2.9
Liability for claims and claim adjustment expenses, net of reinsurance	333.0
Casualty Insurance
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expense, Net	864.5
Cumulative paid claims and allocated claim adjustment expense, Net	262.8
All outstanding liabilities for 2012 and subsequent years, net of reinsurance	601.7
All outstanding liabilities before 2012, net of reinsurance	43.0
Liability for claims and claim adjustment expenses, net of reinsurance	644.7
Marine, Aviation and Energy Insurance
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expense, Net	1,699.9
Cumulative paid claims and allocated claim adjustment expense, Net	1,025.4
All outstanding liabilities for 2012 and subsequent years, net of reinsurance	674.5
All outstanding liabilities before 2012, net of reinsurance	61.4
Liability for claims and claim adjustment expenses, net of reinsurance	735.9
Financial and Professional Insurance
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expense, Net	922.4
Cumulative paid claims and allocated claim adjustment expense, Net	377.4
All outstanding liabilities for 2012 and subsequent years, net of reinsurance	545.0
All outstanding liabilities before 2012, net of reinsurance	27.4
Liability for claims and claim adjustment expenses, net of reinsurance	572.4
Property Catastrophe and Other Property Reinsurance
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expense, Net	1,637.8
Cumulative paid claims and allocated claim adjustment expense, Net	956.2
All outstanding liabilities for 2012 and subsequent years, net of reinsurance	681.6
All outstanding liabilities before 2012, net of reinsurance	47.9
Liability for claims and claim adjustment expenses, net of reinsurance	729.5
Casualty Reinsurance
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expense, Net	1,372.6
Cumulative paid claims and allocated claim adjustment expense, Net	355.7
All outstanding liabilities for 2012 and subsequent years, net of reinsurance	1,016.9
All outstanding liabilities before 2012, net of reinsurance	575.6
Liability for claims and claim adjustment expenses, net of reinsurance	1,592.5
Specialty Reinsurance
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expense, Net	1,222.6
Cumulative paid claims and allocated claim adjustment expense, Net	702.3
All outstanding liabilities for 2012 and subsequent years, net of reinsurance	520.3
All outstanding liabilities before 2012, net of reinsurance	55.4
Liability for claims and claim adjustment expenses, net of reinsurance	575.7
Accident Year 2012 | Property Insurance
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expense, Net	154.7	$ 160.6	$ 166.2	$ 167.3	$ 168.6	$ 170.7
Incurred but Not Reported (IBNR) Claims Liability, Net	$ 0.3
Number of Reported Claims | Integer	6,872
Cumulative paid claims and allocated claim adjustment expense, Net	$ 154.9	157.5	153.4	139.2	129.6	41.5
Accident Year 2012 | Casualty Insurance
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expense, Net	67.2	69.6	61.7	70.5	63.4	78.6
Incurred but Not Reported (IBNR) Claims Liability, Net	$ 5.2
Number of Reported Claims | Integer	3,536
Cumulative paid claims and allocated claim adjustment expense, Net	$ 49.7	41.3	30.0	14.3	6.7	1.3
Accident Year 2012 | Marine, Aviation and Energy Insurance
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expense, Net	329.0	333.0	347.3	326.7	307.2	269.8
Incurred but Not Reported (IBNR) Claims Liability, Net	$ 33.8
Number of Reported Claims | Integer	4,593
Cumulative paid claims and allocated claim adjustment expense, Net	$ 251.7	240.8	212.0	175.7	133.2	52.2
Accident Year 2012 | Financial and Professional Insurance
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expense, Net	90.0	94.5	97.6	94.0	90.1	88.8
Incurred but Not Reported (IBNR) Claims Liability, Net	$ 9.7
Number of Reported Claims | Integer	593
Cumulative paid claims and allocated claim adjustment expense, Net	$ 70.7	65.4	59.5	50.8	39.6	22.8
Accident Year 2012 | Property Catastrophe and Other Property Reinsurance
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expense, Net	280.4	283.4	280.0	287.2	304.2	281.3
Incurred but Not Reported (IBNR) Claims Liability, Net	$ 19.3
Number of Reported Claims | Integer	797
Cumulative paid claims and allocated claim adjustment expense, Net	$ 228.6	217.5	209.9	189.5	136.2	35.7
Accident Year 2012 | Casualty Reinsurance
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expense, Net	235.6	234.8	238.0	246.6	235.2	236.9
Incurred but Not Reported (IBNR) Claims Liability, Net	$ 66.1
Number of Reported Claims | Integer	2,173
Cumulative paid claims and allocated claim adjustment expense, Net	$ 118.5	97.1	65.8	42.2	17.9	2.2
Accident Year 2012 | Specialty Reinsurance
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expense, Net	176.3	176.0	177.4	191.1	201.7	178.5
Incurred but Not Reported (IBNR) Claims Liability, Net	$ 10.5
Number of Reported Claims | Integer	739
Cumulative paid claims and allocated claim adjustment expense, Net	$ 152.2	146.9	141.4	131.2	95.8	$ 25.7
Accident Year 2013 | Property Insurance
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expense, Net	114.3	113.3	117.5	117.9	130.8
Incurred but Not Reported (IBNR) Claims Liability, Net	$ 4.9
Number of Reported Claims | Integer	5,100
Cumulative paid claims and allocated claim adjustment expense, Net	$ 106.4	101.8	89.7	76.4	39.2
Accident Year 2013 | Casualty Insurance
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expense, Net	104.4	121.4	115.9	117.5	134.1
Incurred but Not Reported (IBNR) Claims Liability, Net	$ 13.6
Number of Reported Claims | Integer	3,852
Cumulative paid claims and allocated claim adjustment expense, Net	$ 69.0	53.5	39.8	26.0	2.3
Accident Year 2013 | Marine, Aviation and Energy Insurance
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expense, Net	333.3	326.3	342.9	334.2	321.4
Incurred but Not Reported (IBNR) Claims Liability, Net	$ 17.4
Number of Reported Claims | Integer	4,979
Cumulative paid claims and allocated claim adjustment expense, Net	$ 265.0	235.7	205.3	131.8	41.6
Accident Year 2013 | Financial and Professional Insurance
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expense, Net	101.6	102.3	105.6	101.2	106.6
Incurred but Not Reported (IBNR) Claims Liability, Net	$ 15.2
Number of Reported Claims | Integer	603
Cumulative paid claims and allocated claim adjustment expense, Net	$ 64.5	66.0	31.6	21.5	8.1
Accident Year 2013 | Property Catastrophe and Other Property Reinsurance
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expense, Net	180.8	182.3	193.9	203.0	220.5
Incurred but Not Reported (IBNR) Claims Liability, Net	$ 4.4
Number of Reported Claims | Integer	945
Cumulative paid claims and allocated claim adjustment expense, Net	$ 166.8	162.0	149.7	100.3	34.9
Accident Year 2013 | Casualty Reinsurance
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expense, Net	208.4	225.8	228.4	232.9	217.5
Incurred but Not Reported (IBNR) Claims Liability, Net	$ 62.2
Number of Reported Claims | Integer	2,034
Cumulative paid claims and allocated claim adjustment expense, Net	$ 93.7	65.5	43.2	16.0	3.5
Accident Year 2013 | Specialty Reinsurance
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expense, Net	121.7	122.5	134.5	141.9	146.9
Incurred but Not Reported (IBNR) Claims Liability, Net	$ 7.7
Number of Reported Claims | Integer	620
Cumulative paid claims and allocated claim adjustment expense, Net	$ 103.5	96.4	89.1	72.7	$ 25.4
Accident Year 2014 | Property Insurance
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expense, Net	135.4	134.7	157.9	166.2
Incurred but Not Reported (IBNR) Claims Liability, Net	$ 3.0
Number of Reported Claims | Integer	8,779
Cumulative paid claims and allocated claim adjustment expense, Net	$ 124.2	114.5	87.0	40.5
Accident Year 2014 | Casualty Insurance
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expense, Net	130.9	140.5	128.6	146.4
Incurred but Not Reported (IBNR) Claims Liability, Net	$ 20.4
Number of Reported Claims | Integer	4,312
Cumulative paid claims and allocated claim adjustment expense, Net	$ 60.4	33.1	13.6	2.7
Accident Year 2014 | Marine, Aviation and Energy Insurance
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expense, Net	310.9	299.3	314.5	310.1
Incurred but Not Reported (IBNR) Claims Liability, Net	$ 45.3
Number of Reported Claims | Integer	4,768
Cumulative paid claims and allocated claim adjustment expense, Net	$ 210.2	189.5	117.0	53.6
Accident Year 2014 | Financial and Professional Insurance
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expense, Net	121.4	131.3	132.6	136.4
Incurred but Not Reported (IBNR) Claims Liability, Net	$ 35.2
Number of Reported Claims | Integer	852
Cumulative paid claims and allocated claim adjustment expense, Net	$ 72.8	54.1	30.9	3.0
Accident Year 2014 | Property Catastrophe and Other Property Reinsurance
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expense, Net	152.4	164.0	180.4	191.7
Incurred but Not Reported (IBNR) Claims Liability, Net	$ 4.1
Number of Reported Claims | Integer	1,016
Cumulative paid claims and allocated claim adjustment expense, Net	$ 139.4	129.2	102.3	37.8
Accident Year 2014 | Casualty Reinsurance
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expense, Net	214.0	220.8	212.0	208.7
Incurred but Not Reported (IBNR) Claims Liability, Net	$ 84.5
Number of Reported Claims | Integer	1,842
Cumulative paid claims and allocated claim adjustment expense, Net	$ 60.9	38.2	14.1	2.6
Accident Year 2014 | Specialty Reinsurance
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expense, Net	126.0	135.2	143.4	154.4
Incurred but Not Reported (IBNR) Claims Liability, Net	$ 15.6
Number of Reported Claims | Integer	667
Cumulative paid claims and allocated claim adjustment expense, Net	$ 91.3	83.1	57.9	$ 17.0
Accident Year 2015 | Property Insurance
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expense, Net	201.8	207.3	241.9
Incurred but Not Reported (IBNR) Claims Liability, Net	$ 4.7
Number of Reported Claims | Integer	10,431
Cumulative paid claims and allocated claim adjustment expense, Net	$ 172.1	144.0	57.5
Accident Year 2015 | Casualty Insurance
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expense, Net	188.0	225.5	205.5
Incurred but Not Reported (IBNR) Claims Liability, Net	$ 64.7
Number of Reported Claims | Integer	4,954
Cumulative paid claims and allocated claim adjustment expense, Net	$ 57.0	17.2	3.2
Accident Year 2015 | Marine, Aviation and Energy Insurance
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expense, Net	283.3	301.0	297.9
Incurred but Not Reported (IBNR) Claims Liability, Net	$ 37.9
Number of Reported Claims | Integer	4,356
Cumulative paid claims and allocated claim adjustment expense, Net	$ 174.9	123.7	45.1
Accident Year 2015 | Financial and Professional Insurance
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expense, Net	187.2	177.2	175.9
Incurred but Not Reported (IBNR) Claims Liability, Net	$ 76.7
Number of Reported Claims | Integer	1,138
Cumulative paid claims and allocated claim adjustment expense, Net	$ 70.4	43.7	13.9
Accident Year 2015 | Property Catastrophe and Other Property Reinsurance
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expense, Net	181.6	191.4	219.4
Incurred but Not Reported (IBNR) Claims Liability, Net	$ 18.7
Number of Reported Claims | Integer	1,135
Cumulative paid claims and allocated claim adjustment expense, Net	$ 130.0	96.9	36.2
Accident Year 2015 | Casualty Reinsurance
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expense, Net	214.6	205.3	197.9
Incurred but Not Reported (IBNR) Claims Liability, Net	$ 103.0
Number of Reported Claims | Integer	1,648
Cumulative paid claims and allocated claim adjustment expense, Net	$ 39.2	18.5	3.6
Accident Year 2015 | Specialty Reinsurance
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expense, Net	168.8	174.3	169.7
Incurred but Not Reported (IBNR) Claims Liability, Net	$ 29.4
Number of Reported Claims | Integer	826
Cumulative paid claims and allocated claim adjustment expense, Net	$ 106.5	58.1	$ 18.1
Accident Year 2016 | Property Insurance
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expense, Net	250.7	239.9
Incurred but Not Reported (IBNR) Claims Liability, Net	$ 14.7
Number of Reported Claims | Integer	9,514
Cumulative paid claims and allocated claim adjustment expense, Net	$ 169.7	67.3
Accident Year 2016 | Casualty Insurance
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expense, Net	190.6	219.4
Incurred but Not Reported (IBNR) Claims Liability, Net	$ 117.9
Number of Reported Claims | Integer	5,084
Cumulative paid claims and allocated claim adjustment expense, Net	$ 23.1	4.2
Accident Year 2016 | Marine, Aviation and Energy Insurance
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expense, Net	231.9	219.4
Incurred but Not Reported (IBNR) Claims Liability, Net	$ 49.5
Number of Reported Claims | Integer	4,572
Cumulative paid claims and allocated claim adjustment expense, Net	$ 83.2	30.9
Accident Year 2016 | Financial and Professional Insurance
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expense, Net	213.6	193.0
Incurred but Not Reported (IBNR) Claims Liability, Net	$ 121.1
Number of Reported Claims | Integer	1,365
Cumulative paid claims and allocated claim adjustment expense, Net	$ 71.7	15.4
Accident Year 2016 | Property Catastrophe and Other Property Reinsurance
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expense, Net	277.8	277.9
Incurred but Not Reported (IBNR) Claims Liability, Net	$ 35.1
Number of Reported Claims | Integer	1,297
Cumulative paid claims and allocated claim adjustment expense, Net	$ 167.2	58.0
Accident Year 2016 | Casualty Reinsurance
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expense, Net	250.5	237.7
Incurred but Not Reported (IBNR) Claims Liability, Net	$ 156.2
Number of Reported Claims | Integer	1,151
Cumulative paid claims and allocated claim adjustment expense, Net	$ 34.3	9.6
Accident Year 2016 | Specialty Reinsurance
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expense, Net	244.6	243.2
Incurred but Not Reported (IBNR) Claims Liability, Net	$ 49.5
Number of Reported Claims | Integer	844
Cumulative paid claims and allocated claim adjustment expense, Net	$ 153.1	$ 59.6
Accident Year 2017 | Property Insurance
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expense, Net	297.7
Incurred but Not Reported (IBNR) Claims Liability, Net	$ 51.3
Number of Reported Claims | Integer	6,249
Cumulative paid claims and allocated claim adjustment expense, Net	$ 97.2
Accident Year 2017 | Casualty Insurance
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expense, Net	183.4
Incurred but Not Reported (IBNR) Claims Liability, Net	$ 153.0
Number of Reported Claims | Integer	4,447
Cumulative paid claims and allocated claim adjustment expense, Net	$ 3.6
Accident Year 2017 | Marine, Aviation and Energy Insurance
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expense, Net	211.5
Incurred but Not Reported (IBNR) Claims Liability, Net	$ 70.0
Number of Reported Claims | Integer	3,993
Cumulative paid claims and allocated claim adjustment expense, Net	$ 40.4
Accident Year 2017 | Financial and Professional Insurance
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expense, Net	208.6
Incurred but Not Reported (IBNR) Claims Liability, Net	$ 159.4
Number of Reported Claims | Integer	1,743
Cumulative paid claims and allocated claim adjustment expense, Net	$ 27.3
Accident Year 2017 | Property Catastrophe and Other Property Reinsurance
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expense, Net	564.8
Incurred but Not Reported (IBNR) Claims Liability, Net	$ 216.1
Number of Reported Claims | Integer	1,366
Cumulative paid claims and allocated claim adjustment expense, Net	$ 124.2
Accident Year 2017 | Casualty Reinsurance
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expense, Net	249.5
Incurred but Not Reported (IBNR) Claims Liability, Net	$ 215.9
Number of Reported Claims | Integer	476
Cumulative paid claims and allocated claim adjustment expense, Net	$ 9.1
Accident Year 2017 | Specialty Reinsurance
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expense, Net	385.2
Incurred but Not Reported (IBNR) Claims Liability, Net	$ 211.6
Number of Reported Claims | Integer	763
Cumulative paid claims and allocated claim adjustment expense, Net	$ 95.7

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- Definition

Undiscounted subtotal amount, after reinsurance, of the liability for unpaid claim adjustment expense for short durations insurance contracts not separately presented in claim development information.

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Amount, after reinsurance, of paid claims and allocated claim adjustment expense used in claims development for short-duration insurance contracts. Excludes unallocated claim adjustment expense.

+ References

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 40
-Section 50
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Undiscounted amount, after reinsurance, of incurred-but-not-reported (IBNR) liabilities plus expected development on reported claims, for claims and allocated claim adjustment expense for short-duration insurance contracts. Excludes unallocated claim adjustment expense.

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Undiscounted amount, after reinsurance, of incurred claims and allocated claim adjustment expense used in claims development for short-duration insurance contracts. Excludes unallocated claim adjustment expense.

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Undiscounted amount, after reinsurance, of the liability for unpaid claims and allocated claim adjustment expense for short-duration insurance contracts. Excludes unallocated claim adjustment expense.

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-Publisher FASB
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Undiscounted amount, after reinsurance, of the liability for unpaid claims and allocated claim adjustment expense for short-duration insurance contracts for accident years not separately presented in claim development information. Excludes unallocated claim adjustment expense.

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-Publisher FASB
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