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

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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 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 investment in equity security measured at fair value with change in fair value recognized in net income (FV-NI).

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Amount of investment in equity security without readily determinable fair value.

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Amount of impairment loss on investment in equity security without readily determinable fair value.

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Amount of gain from upward price adjustment on investment in equity security without readily determinable fair value.

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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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Amount of realized gain (loss) on investment in marketable security, including other-than-temporary impairment (OTTI).

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Amount of realized gain (loss) on investment in marketable security, excluding other-than-temporary impairment (OTTI).

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Investment in short-term money-market instruments (such as commercial paper, banker's acceptances, repurchase agreements, government securities, certificates of deposit, and so forth) which are highly liquid (that is, readily convertible to known amounts of cash) and so near their maturity that they present an insignificant risk of changes in value because of changes in interest rates. Generally, only investments with original maturities of three months or less qualify as cash equivalents by definition. Original maturity means an original maturity to the entity holding the investment. For example, both a three-month US Treasury bill and a three-year Treasury note purchased three months from maturity qualify as cash equivalents. However, a Treasury note purchased three-years ago does not become a cash equivalent when its remaining maturity is three months.

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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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Amount of cash outflow for purchase of marketable security.

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The cash outflow associated with extending a long-term loan to a related party. Alternate caption: Payments for Advances to Affiliates.

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The cash inflow associated with the aggregate amount received by the entity through sale or maturity of marketable securities (held-to-maturity or available-for-sale) during the period.

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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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Investments (Narrative) (Details)	3 Months Ended										12 Months Ended
Investments (Narrative) (Details)	Dec. 31, 2018 USD ($) shares	Sep. 30, 2018 USD ($)	Jun. 30, 2018 USD ($)	Jun. 30, 2018 GBP (£)	Mar. 31, 2018 USD ($)	Dec. 31, 2017 USD ($) shares	Sep. 30, 2017 USD ($)	Jun. 30, 2017 USD ($)	Mar. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2018 USD ($) shares	Dec. 31, 2017 USD ($) shares	Dec. 31, 2016 USD ($)	Dec. 31, 2018 GBP (£) shares	Dec. 31, 2017 GBP (£) shares
Schedule of Investments [Line Items]
Money market investments	$ 75,500,000					$ 217,600,000					$ 75,500,000	$ 217,600,000
Marketable equity securities	496,390,000										496,390,000
Marketable equity securities						536,315,000						536,315,000
Cash paid for new marketable equity securities											42,700,000	0	$ 48,300,000
Marketable equity securities purchased													47,900,000
Cumulative realized gain (loss) on marketable securities											0	0	(1,791,000)
Proceeds from sales of marketable equity securities											66,700,000	0	29,700,000
Loss on sales of marketable equity securities													8,100,000
Gain on sales of marketable equity securities											37,300,000		6,200,000
Equity Method Investment, Other than Temporary Impairment						2,800,000
Equity in earnings (losses) of affiliates, net	1,426,000	$ 9,537,000	$ 931,000		$ 2,579,000	(4,697,000)	$ (532,000)	$ 1,331,000	$ 649,000		14,473,000	(3,249,000)	(7,937,000)
Equity Securities without Readily Determinable Fair Value, Amount	30,600,000					$ 19,900,000					30,600,000	19,900,000
Gains on cost method investments	$ 3,200,000	8,500,000									11,663,000	$ 0	$ 0
Impairment loss on cost method investments											$ 2,697,000
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
Marketable equity securities	$ 29,100,000										$ 29,100,000
Marketable equity securities						$ 31,900,000						$ 31,900,000
Intersection [Member]
Schedule of Investments [Line Items]
Ownership percentage of investment in affiliate	11.00%										11.00%			11.00%
HomeHero [Member]
Schedule of Investments [Line Items]
Equity Method Investment, Other than Temporary Impairment										$ 8,400,000
Equity in earnings (losses) of affiliates, net		2,100,000
One Investment [Member]
Schedule of Investments [Line Items]
Equity in earnings (losses) of affiliates, net		$ 5,800,000
Graham Healthcare Group [Member]
Schedule of Investments [Line Items]
Revenue from Related Parties											$ 12,100,000	$ 18,300,000
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%
KIHL [Member] \| York Joint Venture [Member]
Schedule of Investments [Line Items]
Ownership percentage of investment in affiliate	45.00%										45.00%			45.00%
Loan commitment to affiliate \| £														£ 25,000,000
Advances to Affiliate \| £														£ 22,000,000	£ 16,000,000
Payments to Fund Long-term Loans to Related Parties \| £				£ 6,000,000
Loan Receivable, Payment Term	25 years
Loan Receivable Fixed Interest Rate	7.00%