Equity (Balance of and Changes in Each Component of AOCI) (Details) - USD ($) $ in Thousands		6 Months Ended
		Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017	Dec. 31, 2016
Accumulated other comprehensive income		$ 1,004,971	$ 1,671,898	$ 2,063,631	$ 1,139,329
Other Comprehensive Income (Loss), before Reclassifications, before Tax		(1,374,408)	760,556
Reclassification from Accumulated Other Comprehensive Income, Current Period, before Tax		56,012	(36,425)
Other Comprehensive Income (Loss), Tax		(259,736)	191,562
Accumulated Translation Adjustment [Member]
Accumulated other comprehensive income		(142,187)	(151,189)	(86,350)	(172,541)
Other Comprehensive Income (Loss), before Reclassifications, before Tax		(44,227)	(13,936)
Reclassification from Accumulated Other Comprehensive Income, Current Period, before Tax		0	0
Other Comprehensive Income (Loss), Tax		(11,610)	35,288
Accumulated Net Unrealized Investment Gain (Loss) [Member]
Accumulated other comprehensive income	[1]	1,198,338	1,864,477	2,200,661	1,355,033
Other Comprehensive Income (Loss), before Reclassifications, before Tax	[1]	(1,327,195)	774,688
Reclassification from Accumulated Other Comprehensive Income, Current Period, before Tax	[1]	53,646	(39,360)
Other Comprehensive Income (Loss), Tax	[1]	(271,226)	225,884
Accumulated Defined Benefit Plans Adjustment [Member]
Accumulated other comprehensive income		(51,180)	(41,390)	$ (50,680)	$ (43,163)
Other Comprehensive Income (Loss), before Reclassifications, before Tax		(2,986)	(196)
Reclassification from Accumulated Other Comprehensive Income, Current Period, before Tax		2,366	2,935
Other Comprehensive Income (Loss), Tax		$ (120)	$ 966

[1] Includes cash flow hedges of $22,656 and $2,619 as of June 30, 2018 and December 31, 2017, respectively, and $1,131 and $(2,496) as of June 30, 2017 and December 31, 2016, respectively. See Note 5 - “Derivative Instruments” for additional information on cash flow hedges.