Investments (Net Investment Gains (Losses) and Change in Net Unrealized Gains On Available-For-Sale Investments) (Details) - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2018		Dec. 31, 2017		Dec. 31, 2016
Gain (Loss) on Securities [Line Items]
Realized gains		$ 7,106		$ 52,282		$ 76,752
Realized losses		19,080		12,697		28,396
Other-than-temporary impairment losses recognized in net income		0		(7,589)		(18,355)
Net realized investment gains (losses)		(11,974)		39,585		48,356
Change in fair value of equity securities sold during the period	[1]	20,177		6,989		(3,990)
Change in fair value of equity securities held at the end of the period	[1]	(445,799)		(51,877)		20,781
Change in fair value of equity securities	[1]	(425,622)		(44,888)		16,791
Net investment gains (losses)		(437,596)	[2]	(5,303)	[3]	65,147	[4]
Change in net unrealized gains on investments included in other comprehensive income (loss)		(299,446)		1,125,440		342,111
Sales Of Fixed Maturities [Member]
Gain (Loss) on Securities [Line Items]
Realized gains		4,221		5,525		5,160
Realized losses		5,768		1,983		704
Sales Of Equity Securities [Member]
Gain (Loss) on Securities [Line Items]
Realized gains	[1]	0		40,113		70,177
Realized losses	[1]	0		1,830		6,988
Sales Of Short-Term Investments [Member]
Gain (Loss) on Securities [Line Items]
Realized gains		1,604		0		0
Realized losses		(10,545)		(699)		(522)
Other [Member]
Gain (Loss) on Securities [Line Items]
Realized gains		1,281		6,644		1,415
Realized losses		2,767		596		1,827
Fixed Maturities [Member]
Gain (Loss) on Securities [Line Items]
Change in net unrealized gains on investments included in other comprehensive income (loss)		(297,158)		89,741		(56,534)
Equity Securities [Member]
Gain (Loss) on Securities [Line Items]
Change in net unrealized gains on investments included in other comprehensive income (loss)	[1]	0		1,035,793		398,752
Short-Term Investments [Member]
Gain (Loss) on Securities [Line Items]
Change in net unrealized gains on investments included in other comprehensive income (loss)		$ (2,288)		$ (94)		$ (107)

[1] Effective January 1, 2018, the Company adopted ASU No. 2016-01. As a result, equity securities are no longer classified as available-for-sale with unrealized gains and losses recognized in other comprehensive income; rather, all changes in the fair value of equity securities are now recognized in net income. Prior periods have not been restated to conform to the current presentation. See note 1. Prior to adopting ASU No. 2016-01, the Company recorded certain investments in equity securities at estimated fair value with changes in fair value recorded in net income.

[2] Effective January 1, 2018, the Company adopted ASU No. 2016-01. As a result, the change in fair value of equity securities is no longer recognized in other comprehensive income; rather, all changes in the fair value of equity securities are now recognized in net investment losses within net income. Prior periods have not been restated to conform to the current presentation. See note 1.

[3] Effective January 1, 2018, the Company adopted ASU No. 2016-01. As a result, the change in fair value of equity securities is no longer recognized in other comprehensive income; rather, all changes in the fair value of equity securities are now recognized in net investment losses within net income. Prior periods have not been restated to conform to the current presentation. See note 1.

[4] Effective January 1, 2018, the Company adopted ASU No. 2016-01. As a result, the change in fair value of equity securities is no longer recognized in other comprehensive income; rather, all changes in the fair value of equity securities are now recognized in net investment gains within net income. Prior periods have not been restated to conform to the current presentation. See note 1.