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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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Loan Receivable Fixed Interest Rate

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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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Amounts due from parties associated with the reporting entity as evidenced by a written promise to pay, due after 1 year (or 1 business cycle).

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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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Investments (Narrative) (Details) $ in Thousands				3 Months Ended										12 Months Ended
Investments (Narrative) (Details) $ in Thousands	Jul. 01, 2016 GBP (£)	Oct. 01, 2014 USD ($)	Jun. 30, 2014 USD ($) shares	Dec. 31, 2016 USD ($) shares	Sep. 30, 2016 USD ($)	Jun. 30, 2016 USD ($)	Mar. 31, 2016 USD ($)	Dec. 31, 2015 USD ($) shares	Sep. 30, 2015 USD ($)	Jun. 30, 2015 USD ($)	Mar. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)	Jun. 30, 2014 USD ($) shares	Dec. 31, 2016 USD ($) shares	Dec. 31, 2015 USD ($) shares	Dec. 31, 2014 USD ($)	Dec. 31, 2016 GBP (£) shares	Jan. 31, 2015	Sep. 30, 2014	Jul. 03, 2014	Apr. 01, 2014 USD ($)
Schedule of Investments [Line Items]
Commercial paper and money market investments				$ 485,100				$ 433,000						$ 485,100	$ 433,000
Investments in Marketable equity securities				424,229				350,563						424,229	350,563
Cash paid for new marketable equity securities														48,265	145,807	$ 49,998
Marketable equity securities purchased														47,900	146,200
Proceeds from sales of marketable equity securities														29,700		5,800
Marketable Securities, Realized Loss, Excluding Other than Temporary Impairments														(8,100)		(2,600)
Gain on sales of marketable equity securities														6,200
Gain on share exchange														0	0	266,733
Pre-tax gain on equity method investment				$ (7,042)	$ (1,008)	$ (891)	$ 1,004	$ (35)	$ 95	$ (353)	$ (404)			(7,937)	(697)	100,370
Pre-tax gain (loss) on sale of equity method investment														$ 0	$ 4,827	$ 396,553
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
Investments in Marketable equity securities				$ 25,300				$ 24,700						$ 25,300	$ 24,700
HomeHero [Member]
Schedule of Investments [Line Items]
Impairment write-down on a marketable equity security				$ 8,400
Ownership percentage of investment in affiliate				20.00%										20.00%			20.00%
Celtic Healthcare Allegheny Health Network Joint Venture [Member]
Schedule of Investments [Line Items]
Ownership percentage of investment in affiliate																		40.00%
Classified Ventures' sale of apartments.com [Member]
Schedule of Investments [Line Items]
Cash distribution from equity method investment																					$ 95,000
Pre-tax gain on equity method investment													$ 90,900
Classified Ventures LLC [Member]
Schedule of Investments [Line Items]
Ownership percentage of investment in affiliate																			16.50%
Proceeds from sale of equity method investment, including amount held in escrow		$ 408,500
Proceeds from sale of equity method investment								$ 16,500
Pre-tax gain (loss) on sale of equity method investment												$ 396,600
Celtic Healthcare Inc [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%	40.00%
Revenue from Related Parties														$ 14,900
Residential Healthcare Group Inc [Member] \| Residential Home Health Illinois [Member]
Schedule of Investments [Line Items]
Ownership percentage of investment in affiliate				40.00%										40.00%			40.00%			40.00%
Residential Healthcare Group Inc [Member] \| Residential Hospice Illinois [Member]
Schedule of Investments [Line Items]
Ownership percentage of investment in affiliate				42.50%										42.50%			42.50%			42.50%
York University [Member] \| York Joint Venture [Member]
Schedule of Investments [Line Items]
Ownership percentage of investment in affiliate				45.00%										45.00%			45.00%
Accounts Receivable, Related Parties, Noncurrent \| £	£ 25,000,000
Loan Receivable, Payment Term	25 years
Loan receivable, Fixed Rate of Interest	7.00%
Advances to Affiliate \| £																	£ 11,000,000
Berkshire Exchange Transaction [Member] \| Berkshire Hathaway Inc [Member]
Schedule of Investments [Line Items]
Gain on share exchange			$ 266,700
Berkshire Exchange Transaction [Member] \| Berkshire Hathaway Inc [Member] \| Class A Common Stock [Member]
Schedule of Investments [Line Items]
Number of shares exchanged \| shares			2,107										2,107
Berkshire Exchange Transaction [Member] \| Berkshire Hathaway Inc [Member] \| Class B Common Stock [Member]
Schedule of Investments [Line Items]
Number of shares exchanged \| shares			1,278										1,278