Investments (Narrative) (Details) $ in Thousands	12 Months Ended
Dec. 31, 2019 USD ($) securities	Dec. 31, 2018 USD ($) securities
Debt Securities, Available-for-sale [Line Items]
Available-for-sale fixed maturity securities in unrealized loss position, number of positions \| securities	201	1,005
Available-for-sale fixed maturity securities in unrealized loss position, number of positions, 12 months or longer \| securities	122	541
Available-for-sale fixed maturity securities, estimated fair value	$ 963,177	$ 4,594,664
Available-for-sale fixed maturity securities, gross unrealized holding losses	(14,479)	(133,989)
Available-for-sale fixed maturity securities, estimated fair value, 12 months or longer	352,025	1,983,912
Available-for-sale fixed maturity securities, gross unrealized holding losses, 12 months or longer	$ (7,287)	$ (77,504)
Estimated average duration of fixed maturities	5 years 10 months 24 days
Percentage threshold of shareholders' equity used to define concentration of investments	10.00%	10.00%
Ten largest equity holdings	$ 3,000,000
Ten largest equity holdings, percentage of equity portfolio	39.00%
Investments in property and casualty	$ 1,200,000
Investments in property and casualty, percentage of equity portfolio	16.00%
Berkshire Hathaway Inc. [Member]
Debt Securities, Available-for-sale [Line Items]
Investments in property and casualty	$ 721,800
Fixed maturities [Member]
Debt Securities, Available-for-sale [Line Items]
Available-for-sale fixed maturity securities, estimated fair value	959,861	$ 4,397,021
Available-for-sale fixed maturity securities, gross unrealized holding losses	(14,419)	(131,215)
Available-for-sale fixed maturity securities, estimated fair value, 12 months or longer	352,025	1,983,912
Available-for-sale fixed maturity securities, gross unrealized holding losses, 12 months or longer	$ (7,287)	$ (77,504)

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The fair value of investments in the property and casualty insurance industry.

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The percentage of the Company's investment in the property and casualty insurance industry in the equity portfolio.

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Percentage threshold of shareholders' equity used to define concentration of investments.

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The fair value of the ten largest equity holdings of the Company.

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The percentage of the Company's ten largest equity holdings in the equity portfolio.

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Amount of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), in continuous unrealized loss position for more than 12 months, without allowance for credit loss. Includes beneficial interest in securitized financial asset.

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Amount of accumulated unrealized loss on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), in continuous unrealized loss position for 12 months or longer, without allowance for credit loss. Includes beneficial interest in securitized financial asset.

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Number of investments in debt securities measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), in continuous unrealized loss position for 12 months or longer, without an allowance for credit loss. Includes beneficial interest in securitized financial asset.

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Amount of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), in unrealized loss position without allowance for credit loss.

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Number of investments in debt securities measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), in unrealized loss position, without allowance for credit loss. Includes beneficial interest in securitized financial asset.

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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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