Other Comprehensive Income (Loss) (Amounts Reclassified From Accumulated Other Comprehensive Income Into Income, By Component) (Details) - USD ($) $ in Thousands	3 Months Ended																								12 Months Ended
	Dec. 31, 2019	[1]	Sep. 30, 2019	[1]	Jun. 30, 2019	[1]	Mar. 31, 2019	[1]	Dec. 31, 2018	[1]	Sep. 30, 2018	[1]	Jun. 30, 2018	[1]	Mar. 31, 2018	[1]	Dec. 31, 2017	[1]	Sep. 30, 2017	[1]	Jun. 30, 2017	[1]	Mar. 31, 2017	[1]	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Jan. 01, 2018
Reclassification Out Of Accumulated Other Comprehensive Income [Line Items]
Cumulative effect of adoption of ASU No. 2016-01, net of taxes																												$ (2,595,484)
Other-than-temporary impairment losses																									$ 0	$ 0	$ (7,589)
Net realized investment gains (losses), excluding other-than-temporary impairment losses																									(1,482)	(11,974)	47,174
Income (Loss) Before Income Taxes																									2,285,808	(7,855)	87,295
Income Taxes																									(486,346)	(122,498)	313,463
Underwriting, acquisition and insurance expenses																									(1,878,093)	(1,777,511)	(1,589,464)
Net income (loss)	$ 511,513		$ 203,953		$ 506,483		$ 577,513		$ (753,374)		$ 409,028		$ 279,587		$ (65,594)		$ 439,326		$ (261,035)		$ 151,427		$ 71,040		1,799,462	(130,353)	400,758
Cumulative effect of adoption of ASU No 2018-02																												402,853
Unrealized Holding Gains On Available-For-Sale Securities [Member]
Reclassification Out Of Accumulated Other Comprehensive Income [Line Items]
Cumulative effect of adoption of ASU No. 2016-01, net of taxes																												(2,597,976)
Cumulative effect of adoption of ASU No 2018-02																												401,539
Net Actuarial Pension Loss [Member]
Reclassification Out Of Accumulated Other Comprehensive Income [Line Items]
Cumulative effect of adoption of ASU No. 2016-01, net of taxes																												0
Cumulative effect of adoption of ASU No 2018-02																												$ 0
Reclassification Out Of Accumulated Other Comprehensive Income [Member] | Unrealized Holding Gains On Available-For-Sale Securities [Member]
Reclassification Out Of Accumulated Other Comprehensive Income [Line Items]
Other-than-temporary impairment losses																									0	0	(7,589)
Net realized investment gains (losses), excluding other-than-temporary impairment losses																									1,453	(9,935)	41,957
Income (Loss) Before Income Taxes																									1,453	(9,935)	34,368
Income Taxes																									(305)	2,086	(10,072)
Net income (loss)																									1,148	(7,849)	24,296
Reclassification Out Of Accumulated Other Comprehensive Income [Member] | Net Actuarial Pension Loss [Member]
Reclassification Out Of Accumulated Other Comprehensive Income [Line Items]
Income Taxes																									1,348	622	648
Underwriting, acquisition and insurance expenses																									(6,390)	(2,963)	(3,815)
Net income (loss)																									$ (5,042)	$ (2,341)	$ (3,167)

[1] 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 income. Prior periods have not been restated to conform to the current presentation.