Investments (Narrative) (Details) $ in Thousands		12 Months Ended
		Dec. 31, 2018 USD ($) securities	Dec. 31, 2017 USD ($) securities	Dec. 31, 2016 USD ($)
Debt Securities, Available-for-sale [Line Items]
Available-for-sale fixed maturity securities in unrealized loss position, number of positions | securities		1,005
Available-for-sale securities in unrealized loss position, number of positions | securities			739
Available-for-sale fixed maturity securities in unrealized loss position, number of positions, 12 months or longer | securities		541
Available-for-sale fixed maturity securities, estimated fair value		$ 4,594,664
Available-for-sale securities, estimated fair value			$ 2,767,652
Available-for-sale fixed maturity securities, gross unrealized holding losses		(133,989)
Available-for-sale securities, gross unrealized holding losses			$ (57,348)
Number of available-for-sale securities positions in a continuous unrealized loss position for one year or longer at period end | securities			272
Available-for-sale fixed maturity securities, estimated fair value, 12 months or longer		1,983,912
Available-for-sale securities, estimated fair value, 12 months or longer			$ 1,481,341
Available-for-sale fixed maturity securities, gross unrealized holding losses, 12 months or longer		$ (77,504)
Available-for-sale securities, gross unrealized holding losses, 12 months or longer			(34,348)
Estimated average duration of fixed maturities		6 years 1 month
Cumulative credit loss recognized in income		$ 0	$ 0	$ 10,700
Percentage threshold of shareholders' equity used to define concentration of investments		10.00%	10.00%
Ten largest equity holdings		$ 2,300,000
Ten largest equity holdings, percentage of equity portfolio		41.00%
Investments in property and casualty		$ 1,000,000
Investments in property and casualty, percentage of equity portfolio		18.00%
Berkshire Hathaway Inc. [Member]
Debt Securities, Available-for-sale [Line Items]
Investments in property and casualty		$ 645,900
Fixed Maturities [Member]
Debt Securities, Available-for-sale [Line Items]
Available-for-sale fixed maturity securities in unrealized loss position, number of positions, 12 months or longer | securities			258
Available-for-sale fixed maturity securities, estimated fair value		4,397,021	$ 2,246,614
Available-for-sale fixed maturity securities, gross unrealized holding losses		(131,215)	(37,344)
Available-for-sale fixed maturity securities, estimated fair value, 12 months or longer		1,983,912	1,468,807
Available-for-sale fixed maturity securities, gross unrealized holding losses, 12 months or longer		$ (77,504)	(31,054)
Equity Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Available-for-sale securities, estimated fair value	[1]		151,934
Available-for-sale securities, gross unrealized holding losses	[1]		$ (19,935)
Number of available-for-sale securities positions in a continuous unrealized loss position for one year or longer at period end | securities			14
Available-for-sale securities, estimated fair value, 12 months or longer	[1]		$ 12,534
Available-for-sale securities, gross unrealized holding losses, 12 months or longer	[1]		$ (3,294)

[1] Effective January 1, 2018, the Company adopted ASU No. 2016-01 and equity securities are no longer classified as available-for-sale. Prior periods have not been restated to conform to the current presentation. See note 1.