Equity securities classified as available-for-sale securities acquired.

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Unsecured short-term debt instrument issued by corporations which are highly liquid investments that are readily convertible to know amounts of cash and interest-bearing accounts that typically pay a higher interest rate and are FDIC insured.

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Description of the payment terms of the debt instrument (for example whether periodic payments include principal and frequency of payments) and discussion about any contingencies associated with the payment.

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This item represents the total realized loss included in earnings for the period as a result of selling marketable securities categorized as trading, available-for-sale, or held-to-maturity.

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The number of shares held in an investment.

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Long-Term advances receivable from a party that is affiliated with the reporting entity by means of direct or indirect ownership. This does not include advances to clients.

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Amount of equity securities categorized neither as held-to-maturity nor as trading.

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The total amount of proceeds received for the sale of securities, categorized neither as held-to-maturity nor trading securities, during the reporting period.

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This item represents an other than temporary decline in value that has been recognized against an investment accounted for under the equity method of accounting. The excess of the carrying amount over the fair value of the investment represents the amount of the write down which is or was reflected in earnings. The written down value is a new cost basis with the adjusted value of the investment becoming its new carrying value subject to the equity accounting method. Evidence of a loss in value might include, but would not necessarily be limited to, absence of an ability to recover the carrying amount of the investment or inability of the investee to sustain an earnings capacity which would justify the carrying amount of the investment.

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The percentage of ownership of common stock or equity participation in the investee accounted for under the equity method of accounting.

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Amount of gain (loss) on sale or disposal of an equity method investment.

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No definition available.

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This item represents the entity's proportionate share for the period of the net income (loss) of its investee (such as unconsolidated subsidiaries and joint ventures) to which the equity method of accounting is applied. This item includes income or expense related to stock-based compensation based on the investor's grant of stock to employees of an equity method investee.

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This item represents the total realized gain (loss) included in earnings for the period as a result of selling marketable securities categorized as trading, available-for-sale, or held-to-maturity.

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No definition available.

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Minimum amount of other commitment not otherwise specified in the taxonomy. Excludes commitments explicitly modeled in the taxonomy, including but not limited to, long-term and short-term purchase commitments, recorded and unrecorded purchase obligations, supply commitments, registration payment arrangements, leases, debt, product warranties, guarantees, environmental remediation obligations, and pensions.

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The cash outflow to acquire equity securities classified as available-for-sale securities, because they are not classified as trading securities.

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Identify the stated interest rate per the agreement, for example, leasing and debt arrangements between related parties.

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Amount of revenue, fees and commissions earned from transactions between (a) a parent company and its subsidiaries; (b) subsidiaries of a common parent; (c) an entity and trusts for the benefit of employees, for example, but not limited to, pension and profit-sharing trusts that are managed by or under the trusteeship of the entity's management; (d) an entity and its principal, owners, management, or members of their immediate families; and (e) affiliates.

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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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Percentage of subsidiary's or equity method investee's stock owned by parent immediately after all stock transactions.

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Investments (Narrative) (Details) £ in Millions		3 Months Ended								12 Months Ended
Investments (Narrative) (Details) £ in Millions	Jul. 31, 2016 GBP (£)	Dec. 31, 2017 USD ($) shares	Sep. 30, 2017 USD ($)	Jun. 30, 2017 USD ($)	Mar. 31, 2017 USD ($)	Dec. 31, 2016 USD ($) shares	Sep. 30, 2016 USD ($)	Jun. 30, 2016 USD ($)	Mar. 31, 2016 USD ($)	Dec. 31, 2017 USD ($) shares	Dec. 31, 2016 USD ($) shares	Dec. 31, 2015 USD ($)	Dec. 31, 2017 GBP (£) shares	Dec. 31, 2016 GBP (£) shares
Schedule of Investments [Line Items]
Commercial paper and money market investments		$ 217,600,000				$ 485,100,000				$ 217,600,000	$ 485,100,000
Investments in Marketable equity securities		536,315,000				424,229,000				536,315,000	424,229,000
Cash paid for new marketable equity securities										0	48,265,000	$ 145,807,000
Available-for-sale Securities, Gross Realized Gains (Losses), Sale Proceeds										0	29,700,000
Marketable equity securities purchased											47,900,000	146,200,000
Loss on sales of marketable equity securities											8,100,000
Gain on sales of marketable equity securities											6,200,000
Equity Method Investment, Other than Temporary Impairment		2,800,000
Pre-tax gain on equity method investment		$ (4,697,000)	$ (532,000)	$ 1,331,000	$ 649,000	$ (7,042,000)	$ (1,008,000)	$ (891,000)	$ 1,004,000	(3,249,000)	(7,937,000)	(697,000)
Pre-tax gain (loss) on sale of equity method investment										$ 0	$ 0	$ 4,827,000
Markel Corporation [Member]
Schedule of Investments [Line Items]
Number of shares held in investment \| shares		28,000				28,000				28,000	28,000		28,000	28,000
Investments in Marketable equity securities		$ 31,900,000				$ 25,300,000				$ 31,900,000	$ 25,300,000
Intersection [Member]
Schedule of Investments [Line Items]
Equity Method Investment, Ownership Percentage										11.00%
HomeHero [Member]
Schedule of Investments [Line Items]
Equity Method Investment, Other than Temporary Impairment						$ 8,400,000
Graham Healthcare Group [Member]
Schedule of Investments [Line Items]
Revenue from Related Parties										$ 18,300,000	$ 14,900,000
Graham Healthcare Group [Member] \| Residential Home Health Illinois [Member]
Schedule of Investments [Line Items]
Ownership percentage of investment in affiliate		40.00%								40.00%			40.00%
Graham Healthcare Group [Member] \| Residential Hospice Illinois [Member]
Schedule of Investments [Line Items]
Ownership percentage of investment in affiliate		42.50%								42.50%			42.50%
Graham Healthcare Group [Member] \| Residential And Michigan Hospital Joint Venture [Member]
Schedule of Investments [Line Items]
Ownership percentage of investment in affiliate		40.00%								40.00%			40.00%
Graham Healthcare Group [Member] \| Celtic Healthcare Allegheny Health Network Joint Venture [Member]
Schedule of Investments [Line Items]
Ownership percentage of investment in affiliate		40.00%								40.00%			40.00%
York University [Member] \| York Joint Venture [Member]
Schedule of Investments [Line Items]
Ownership percentage of investment in affiliate		45.00%								45.00%			45.00%
Other Commitment \| £	£ 25.0
Loan Receivable, Payment Term	25 years
Related Party Transaction, Rate	7.00%
Advances to Affiliate \| £													£ 5.0	£ 11.0