Equity (Balance of and Changes in Each Component of AOCI) (Details) - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Accumulated Other Comprehensive Income Loss Net Of Tax [Roll Forward]
Balance, beginning of year		$ 1,139,329	$ 708,284	$ 1,657,129
Other Comprehensive Income (Loss), Foreign Currency Transaction and Translation Adjustment, Net of Tax		1,062,087	675,627	(1,349,528)
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax		(29,057)	(63,800)	78,998
Other Comprehensive Income (Loss), Tax		(265,100)	(180,782)	321,685
Balance, end of year		2,063,631	1,139,329	708,284
Accumulated Translation Adjustment [Member]
Accumulated Other Comprehensive Income Loss Net Of Tax [Roll Forward]
Balance, beginning of year		(172,541)	(181,151)	81,847
Other Comprehensive Income (Loss), Foreign Currency Transaction and Translation Adjustment, Net of Tax		26,973	29,691	(246,520)
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax		0	0	0
Other Comprehensive Income (Loss), Tax		42,153	(21,081)	(16,478)
Balance, end of year		(86,350)	(172,541)	(181,151)
Accumulated Net Unrealized Investment Gain (Loss) [Member]
Accumulated Other Comprehensive Income Loss Net Of Tax [Roll Forward]
Balance, beginning of year	[1]	1,355,033	935,697	1,624,773
Other Comprehensive Income (Loss), Foreign Currency Transaction and Translation Adjustment, Net of Tax	[1]	1,039,387	646,887	(1,101,760)
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax	[1]	34,460	69,622	(72,795)
Other Comprehensive Income (Loss), Tax	[1]	(306,849)	(157,929)	339,889
Balance, end of year	[1]	2,200,661	1,355,033	935,697
Accumulated Defined Benefit Plans Adjustment [Member]
Accumulated Other Comprehensive Income Loss Net Of Tax [Roll Forward]
Balance, beginning of year		(43,163)	(46,262)	(49,491)
Other Comprehensive Income (Loss), Foreign Currency Transaction and Translation Adjustment, Net of Tax		(4,273)	(951)	(1,248)
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax		(5,403)	(5,822)	(6,203)
Other Comprehensive Income (Loss), Tax		(404)	(1,772)	(1,726)
Balance, end of year		(50,680)	$ (43,163)	$ (46,262)
Accounting Standards Update 2018 02 [Member]
Accumulated Other Comprehensive Income Loss Net Of Tax [Roll Forward]
New Accounting Pronouncement or Change in Accounting Principle, Effect of Adoption, Quantification		17,723
Accounting Standards Update 2018 02 [Member] | Accumulated Translation Adjustment [Member]
Accumulated Other Comprehensive Income Loss Net Of Tax [Roll Forward]
New Accounting Pronouncement or Change in Accounting Principle, Effect of Adoption, Quantification		17,065
Accounting Standards Update 2018 02 [Member] | Accumulated Net Unrealized Investment Gain (Loss) [Member]
Accumulated Other Comprehensive Income Loss Net Of Tax [Roll Forward]
New Accounting Pronouncement or Change in Accounting Principle, Effect of Adoption, Quantification		147,550
Accounting Standards Update 2018 02 [Member] | Accumulated Defined Benefit Plans Adjustment [Member]
Accumulated Other Comprehensive Income Loss Net Of Tax [Roll Forward]
New Accounting Pronouncement or Change in Accounting Principle, Effect of Adoption, Quantification		(8,243)
Accounting Standards Update 2018 02 [Member] | AOCI Attributable to Parent [Member]
Accumulated Other Comprehensive Income Loss Net Of Tax [Roll Forward]
New Accounting Pronouncement or Change in Accounting Principle, Effect of Adoption, Quantification		$ 156,372

[1] Includes cash flow hedges of $2,619, $(2,496) and $(29,397) as of December 31, 2017, 2016 and 2015, respectively. See Note 5 for additional information on cash flow hedges.