Equity (Balance of and Changes in Each Component of AOCI) (Details) - USD ($) $ in Thousands		3 Months Ended	9 Months Ended
		Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2018	Dec. 31, 2017
Accumulated other comprehensive income		$ 812,805	$ 3,258,346	$ 812,805	$ 636,763	$ 2,063,631
Other Comprehensive Income (Loss), before Reclassifications, before Tax			3,530,571	(1,666,514)
Reclassification from Accumulated Other Comprehensive Income, Current Period, before Tax			(166,803)	83,491
New Accounting Pronouncement or Change in Accounting Principle, Effect of Adoption, Quantification		0	(87)	(2,020)
Other Comprehensive Income (Loss), Tax			(742,185)	334,770
Accumulated Translation Adjustment [Member]
Accumulated other comprehensive income		(119,298)	(157,174)	(119,298)	(168,698)	(86,350)
Other Comprehensive Income (Loss), before Reclassifications, before Tax			16,155	(24,977)
Reclassification from Accumulated Other Comprehensive Income, Current Period, before Tax			0	0
New Accounting Pronouncement or Change in Accounting Principle, Effect of Adoption, Quantification				(2,573)
Other Comprehensive Income (Loss), Tax			4,631	5,398
Accumulated Net Unrealized Investment Gain (Loss) [Member]
Accumulated other comprehensive income	[1]	982,352	3,472,549	982,352	856,159	2,200,661
Other Comprehensive Income (Loss), before Reclassifications, before Tax	[1]		3,526,151	(1,638,146)
Reclassification from Accumulated Other Comprehensive Income, Current Period, before Tax	[1]		(170,531)	79,505
New Accounting Pronouncement or Change in Accounting Principle, Effect of Adoption, Quantification	[1]			0
Other Comprehensive Income (Loss), Tax	[1]		(739,230)	340,332
Accumulated Defined Benefit Plans Adjustment [Member]
Accumulated other comprehensive income		$ (50,249)	(57,029)	(50,249)	$ (50,698)	$ (50,680)
Other Comprehensive Income (Loss), before Reclassifications, before Tax			(11,735)	(3,391)
Reclassification from Accumulated Other Comprehensive Income, Current Period, before Tax			3,728	3,986
New Accounting Pronouncement or Change in Accounting Principle, Effect of Adoption, Quantification				0
Other Comprehensive Income (Loss), Tax			$ 1,676	$ (164)

[1] Includes cash flow hedges of $(47,202) and $8,788 as of September 30, 2019 and December 31, 2018, respectively, and $29,043 and $2,619 as of September 30, 2018 and December 31, 2017, respectively. See Note 5 - “Derivative Instruments” for additional information on cash flow hedges.