Equity (Balance of and Changes in Each Component of AOCI) (Details) - USD ($) $ in Thousands		6 Months Ended
		Jun. 30, 2017	Jun. 30, 2016	Dec. 31, 2016	Dec. 31, 2015
Accumulated other comprehensive income		$ 1,671,898	$ 1,985,374	$ 1,139,329	$ 708,284
Other Comprehensive Income (Loss), before Reclassifications, before Tax		760,556	1,853,044
Reclassification from Accumulated Other Comprehensive Income, Current Period, before Tax		(36,425)	(20,899)
Other Comprehensive Income (Loss), Tax		191,562	555,055
Accumulated Translation Adjustment [Member]
Accumulated other comprehensive income		(151,189)	(93,476)	(172,541)	(181,151)
Other Comprehensive Income (Loss), before Reclassifications, before Tax		(13,936)	99,374
Reclassification from Accumulated Other Comprehensive Income, Current Period, before Tax		0	0
Other Comprehensive Income (Loss), Tax		35,288	(11,699)
Accumulated Net Unrealized Investment Gain (Loss) [Member]
Accumulated other comprehensive income	[1]	1,864,477	2,126,815	1,355,033	935,697
Other Comprehensive Income (Loss), before Reclassifications, before Tax	[1]	774,688	1,759,753
Reclassification from Accumulated Other Comprehensive Income, Current Period, before Tax	[1]	(39,360)	(24,366)
Other Comprehensive Income (Loss), Tax	[1]	225,884	544,269
Accumulated Defined Benefit Plans Adjustment [Member]
Accumulated other comprehensive income		(41,390)	(47,965)	$ (43,163)	$ (46,262)
Other Comprehensive Income (Loss), before Reclassifications, before Tax		(196)	(6,083)
Reclassification from Accumulated Other Comprehensive Income, Current Period, before Tax		2,935	3,467
Other Comprehensive Income (Loss), Tax		$ 966	$ (913)

[1] Includes cash flow hedges of $1,131 and $(2,496) as of June 30, 2017 and December 31, 2016, respectively, and $(41,192) and $(29,397) as of June 30, 2016 and December 31, 2015, respectively. See Note 5 - “Derivative Instruments” for additional information on cash flow hedges.