Equity (Balance of and Changes in Each Component of AOCI) (Details) - USD ($) $ in Thousands		6 Months Ended
		Jun. 30, 2019	Jun. 30, 2018	Dec. 31, 2018	Dec. 31, 2017
Accumulated other comprehensive income		$ 2,642,113	$ 1,004,971	$ 636,763	$ 2,063,631
Other Comprehensive Income (Loss), before Reclassifications, before Tax		2,657,376	(1,374,408)
Reclassification from Accumulated Other Comprehensive Income, Current Period, before Tax		(95,609)	56,012
New Accounting Pronouncement or Change in Accounting Principle, Effect of Adoption, Quantification		(87)	(2,020)
Other Comprehensive Income (Loss), Tax		(556,417)	259,736
Accumulated Translation Adjustment [Member]
Accumulated other comprehensive income		(122,709)	(142,187)	(168,698)	(86,350)
Other Comprehensive Income (Loss), before Reclassifications, before Tax		43,534	(44,227)
Reclassification from Accumulated Other Comprehensive Income, Current Period, before Tax		0	0
Other Comprehensive Income (Loss), Tax		(2,455)	11,610
Accumulated Net Unrealized Investment Gain (Loss) [Member]
Accumulated other comprehensive income	[1]	2,815,893	1,198,338	856,159	2,200,661
Other Comprehensive Income (Loss), before Reclassifications, before Tax	[1]	2,617,211	(1,327,195)
Reclassification from Accumulated Other Comprehensive Income, Current Period, before Tax	[1]	(98,502)	53,646
Other Comprehensive Income (Loss), Tax	[1]	(558,975)	271,226
Accumulated Defined Benefit Plans Adjustment [Member]
Accumulated other comprehensive income		(51,071)	(51,180)	$ (50,698)	$ (50,680)
Other Comprehensive Income (Loss), before Reclassifications, before Tax		(3,369)	(2,986)
Reclassification from Accumulated Other Comprehensive Income, Current Period, before Tax		2,893	2,366
Other Comprehensive Income (Loss), Tax		$ 103	$ 120

[1] Includes cash flow hedges of $(14,862) and $8,788 as of June 30, 2019 and December 31, 2018, respectively, and $22,656 and $2,619 as of June 30, 2018 and December 31, 2017, respectively. See Note 5 - “Derivative Instruments” for additional information on cash flow hedges.