Realized and Unrealized Investment Gains and Losses (Change in Unrealized Gains (Losses) of Available For Sales Securities) (Details) - USD ($)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Jan. 01, 2018	Dec. 31, 2015
Realized and Unrealized Investment Gains Losses [Line Items]
Total change in unrealized gains (losses) before income taxes and noncontrolling interests	$ (302,737,000)	$ (69,425,000)	$ 371,715,000
Income tax benefit (expense)	50,410,000	17,673,000	(125,315,000)
Noncontrolling interests	(46,000)	19,000	59,000
Total change in unrealized gains and losses	(252,373,000)	(51,733,000)	246,459,000
Investment funds	(212,000)	125,423,000	58,861,000
Gain on sale of other investments	1,977,000	20,141,000	138,519,000
Other-than-temporary impairments	5,687,000	0	18,114,000
Retained earnings
Realized and Unrealized Investment Gains Losses [Line Items]
Cumulative effect adjustment resulting from changes in accounting principles		215,939,000	0		$ 0
Accounting Standards Update 2016-01 | Retained earnings
Realized and Unrealized Investment Gains Losses [Line Items]
Cumulative effect adjustment resulting from changes in accounting principles				$ 291,000,000
Fixed maturity securities
Realized and Unrealized Investment Gains Losses [Line Items]
Total change in unrealized gains (losses) before income taxes and noncontrolling interests	(297,084,000)	(2,192,000)	(107,094,000)
Other-than-temporary impairments	6,000,000
Previously impaired fixed maturity securities
Realized and Unrealized Investment Gains Losses [Line Items]
Total change in unrealized gains (losses) before income taxes and noncontrolling interests	(132,000)	895,000	451,000
Equity securities
Realized and Unrealized Investment Gains Losses [Line Items]
Total change in unrealized gains (losses) before income taxes and noncontrolling interests	0	(77,971,000)	465,727,000
Other-than-temporary impairments			18,000,000
Investment funds
Realized and Unrealized Investment Gains Losses [Line Items]
Total change in unrealized gains (losses) before income taxes and noncontrolling interests	$ (5,521,000)	9,843,000	12,631,000
Office Building in Washington, D.C.
Realized and Unrealized Investment Gains Losses [Line Items]
Investment funds		$ 124,000,000
Aero Precision Industries and Certain Related Aviation Services
Realized and Unrealized Investment Gains Losses [Line Items]
Gain on sale of other investments			$ 135,000,000