Equity (Balance of and Changes in Each Component of AOCI) (Details) - USD ($) $ in Thousands		9 Months Ended		12 Months Ended
		Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017	Dec. 31, 2016
Accumulated other comprehensive income		$ 812,805	$ 1,625,757	$ 2,063,631	$ 1,139,329
Other Comprehensive Income (Loss), before Reclassifications, before Tax		(1,666,514)	694,490
Reclassification from Accumulated Other Comprehensive Income, Current Period, before Tax		83,491	(47,401)
New Accounting Pronouncement or Change in Accounting Principle, Effect of Adoption, Quantification		2,573		156,400
Other Comprehensive Income (Loss), Tax		334,770	(160,661)
Accumulated Translation Adjustment [Member]
Accumulated other comprehensive income		(119,298)	(104,456)	(86,350)	(172,541)
Other Comprehensive Income (Loss), before Reclassifications, before Tax		(24,977)	23,117
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		5,398	(44,968)
Accumulated Net Unrealized Investment Gain (Loss) [Member]
Accumulated other comprehensive income	[1]	982,352	1,770,903	2,200,661	1,355,033
Other Comprehensive Income (Loss), before Reclassifications, before Tax	[1]	(1,638,146)	671,564
Reclassification from Accumulated Other Comprehensive Income, Current Period, before Tax	[1]	79,505	(51,407)
New Accounting Pronouncement or Change in Accounting Principle, Effect of Adoption, Quantification	[1]	0
Other Comprehensive Income (Loss), Tax	[1]	340,332	(204,287)
Accumulated Defined Benefit Plans Adjustment [Member]
Accumulated other comprehensive income		(50,249)	(40,690)	$ (50,680)	$ (43,163)
Other Comprehensive Income (Loss), before Reclassifications, before Tax		(3,391)	(191)
Reclassification from Accumulated Other Comprehensive Income, Current Period, before Tax		3,986	4,006
New Accounting Pronouncement or Change in Accounting Principle, Effect of Adoption, Quantification		0
Other Comprehensive Income (Loss), Tax		$ (164)	$ (1,342)

[1] Includes cash flow hedges of $29,043 and $2,619 as of September 30, 2018 and December 31, 2017, respectively, and $347 and $(2,496) as of September 30, 2017 and December 31, 2016, respectively. See Note 5 - “Derivative Instruments” for additional information on cash flow hedges.