Commitments and Contingencies - Narrative (Details) - USD ($) $ in Thousands	12 Months Ended
Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Other Commitments [Line Items]
Additional investment commitment - other investments	$ 82,600
Commitment To Invest In Limited Partnerships Time Period	6 years 6 months
Commitment To Invest In Closed End Funds Limited Partnerships And Investment Vehicles	$ 308,000
Commitment To Invest In Closed End Funds Limited Partnerships Time Period	5 years 1 month 6 days
Operating Leases, Rent Expense, Net	$ 76,300	$ 68,000	$ 71,100
Sale Leaseback Transaction, Net Book Value	55,700	55,500
Sale Leaseback Transaction, Amount Due under Financing Arrangement	94,200	89,900
Sale Leaseback Transaction, Deferred Gain, Net	16,900
Total minimum future lease payments	$ 133,377
Branch Profit Tax On Taxable Income Percentage	30.00%
Letter of credit facility
Other Commitments [Line Items]
Available letter of credit facilities - in use	$ 2,549,226	$ 2,345,293
Collateralized by certain assets of the Company's investment portfolio	54.20%	48.60%
Shareholders' equity \| Credit concentration risk
Other Commitments [Line Items]
Maximum percentage of ceded written premium	5.00%
Property and casualty operations \| Gross written premium \| Credit concentration risk
Other Commitments [Line Items]
Maximum percentage of ceded written premium	10.00%

For an entity that discloses a concentration risk in relation to quantitative amount, which serves as the "benchmark" (or denominator) in the equation, this concept represents the concentration percentage derived from the division.

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-Name Accounting Standards Codification
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The carrying value as of the balance sheet date of the current and noncurrent portions of long-term obligations drawn from a line of credit, which is a bank's commitment to make loans up to a specific amount. Examples of items that might be included in the application of this element may consist of letters of credit, standby letters of credit, and revolving credit arrangements, under which borrowings can be made up to a maximum amount as of any point in time conditional on satisfaction of specified terms before, as of and after the date of drawdowns on the line. Includes short-term obligations that would normally be classified as current liabilities but for which (a) postbalance sheet date issuance of a long term obligation to refinance the short term obligation on a long term basis, or (b) the enterprise has entered into a financing agreement that clearly permits the enterprise to refinance the short-term obligation on a long term basis and the following conditions are met (1) the agreement does not expire within 1 year and is not cancelable by the lender except for violation of an objectively determinable provision, (2) no violation exists at the BS date, and (3) the lender has entered into the financing agreement is expected to be financially capable of honoring the agreement.

+ References

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Amount of minimum lease payments for sale-leaseback transactions accounted for using the deposit method or as a financing.

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Rental expense for the reporting period incurred under operating leases, including minimum and any contingent rent expense, net of related sublease income.

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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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The liability due as of the balance sheet date for leasebacks accounted for using the deposit method.

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The portion of the gain that will be recorded as income or a reduction in rent expense in future periods less amounts recognized in the current period.

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The net book value of the asset(s) sold in connection with the sale of the property to another party and lease back to the seller.

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- Definition

Potential maximum branch profits tax on deemed US based income.

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- Definition

Commitment to invest in closed end funds, limited partnerships and investment vehicles.

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Commitment to invest in limited partnerships and provide liquidity financing to structured investment vehicle including potential additional add-on clauses.

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- Definition

Percentage of Letters of credit collateralized by Investments.

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