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Reserves for Losses and Loss Adjustment Expenses - Short-duration Insurance Contracts, Claims Development by Accident Year (Details) $ in Millions	Mar. 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 ($)
Claims Development [Line Items]
Liability for claims and claims adjustment expenses, net of reinsurance	$ 5,021.0
Insurance
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expense, Net	4,793.1
Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	2,710.1
All outstanding liabilities for 2012 and subsequent years, net of reinsurance	2,083.0
All outstanding liabilities before 2012, net of reinsurance	131.1
Liability for claims and claims adjustment expenses, net of reinsurance	2,214.1
Reinsurance
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expense, Net	4,418.8
Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	2,271.9
All outstanding liabilities for 2012 and subsequent years, net of reinsurance	2,146.9
All outstanding liabilities before 2012, net of reinsurance	660.0
Liability for claims and claims adjustment expenses, net of reinsurance	2,806.9
Accident Year 2012 | Insurance
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expense, Net	644.3	$ 644.4	$ 661.1	$ 676.3	$ 661.7	$ 632.4	$ 611.2
Incurred but Not Reported (IBNR) Claims Liability, Net	$ 29.3
Number of Reported Claims | Claim	15,648
Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	$ 532.0	530.2	508.0	457.5	382.2	310.6	118.5
Accident Year 2012 | Reinsurance
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expense, Net	692.2	695.1	696.9	698.0	727.2	743.7	699.0
Incurred but Not Reported (IBNR) Claims Liability, Net	$ 79.7
Number of Reported Claims | Claim	3,791
Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	$ 508.0	501.5	463.4	419.2	364.8	251.4	$ 64.0
Accident Year 2013 | Insurance
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expense, Net	654.1	657.0	667.0	685.3	674.4	696.7
Incurred but Not Reported (IBNR) Claims Liability, Net	$ 45.1
Number of Reported Claims | Claim	14,597
Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	$ 524.1	507.9	459.4	368.5	257.3	92.1
Accident Year 2013 | Reinsurance
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expense, Net	513.2	514.3	533.9	560.5	581.4	588.2
Incurred but Not Reported (IBNR) Claims Liability, Net	$ 72.6
Number of Reported Claims | Claim	3,641
Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	$ 373.3	366.6	326.3	284.0	190.9	$ 64.2
Accident Year 2014 | Insurance
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expense, Net	705.4	702.6	709.6	737.6	763.0
Incurred but Not Reported (IBNR) Claims Liability, Net	$ 70.8
Number of Reported Claims | Claim	18,855
Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	$ 479.1	470.4	393.1	249.8	100.2
Accident Year 2014 | Reinsurance
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expense, Net	486.4	495.7	523.1	539.1	557.6
Incurred but Not Reported (IBNR) Claims Liability, Net	$ 94.9
Number of Reported Claims | Claim	3,614
Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	$ 302.1	293.7	252.3	175.5	$ 57.8
Accident Year 2015 | Insurance
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expense, Net	871.2	867.2	918.0	927.6
Incurred but Not Reported (IBNR) Claims Liability, Net	$ 162.7
Number of Reported Claims | Claim	21,321
Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	$ 508.4	478.2	331.4	120.4
Accident Year 2015 | Reinsurance
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expense, Net	562.4	568.1	574.7	590.3
Incurred but Not Reported (IBNR) Claims Liability, Net	$ 142.0
Number of Reported Claims | Claim	3,770
Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	$ 297.9	276.9	174.6	$ 58.1
Accident Year 2016 | Insurance
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expense, Net	887.7	892.3	920.4
Incurred but Not Reported (IBNR) Claims Liability, Net	$ 300.0
Number of Reported Claims | Claim	21,042
Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	$ 399.9	349.9	118.7
Accident Year 2016 | Reinsurance
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expense, Net	775.0	775.5	761.4
Incurred but Not Reported (IBNR) Claims Liability, Net	$ 234.8
Number of Reported Claims | Claim	3,492
Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	$ 372.4	355.5	$ 126.9
Accident Year 2017 | Insurance
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expense, Net	870.6	907.0
Incurred but Not Reported (IBNR) Claims Liability, Net	$ 232.7
Number of Reported Claims | Claim	19,494
Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	$ 261.6	169.4
Accident Year 2017 | Reinsurance
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expense, Net	1,213.1	1,204.4
Incurred but Not Reported (IBNR) Claims Liability, Net	$ 483.7
Number of Reported Claims | Claim	3,240
Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	$ 415.8	$ 229.3
Accident Year 2018 | Insurance
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expense, Net	159.8
Incurred but Not Reported (IBNR) Claims Liability, Net	$ 113.8
Number of Reported Claims | Claim	2,636
Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	$ 5.0
Accident Year 2018 | Reinsurance
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expense, Net	176.5
Incurred but Not Reported (IBNR) Claims Liability, Net	$ 148.0
Number of Reported Claims | Claim	1,010
Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	$ 2.4

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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.

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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 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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