Equity (Balance of and Changes in Each Component of AOCI) (Details) - USD ($) $ in Thousands		9 Months Ended
		Sep. 30, 2017	Sep. 30, 2016	Dec. 31, 2016	Dec. 31, 2015
Accumulated other comprehensive income		$ 1,625,757	$ 2,212,326	$ 1,139,329	$ 708,284
Other Comprehensive Income (Loss), before Reclassifications, before Tax		694,490	2,254,015
Reclassification from Accumulated Other Comprehensive Income, Current Period, before Tax		(47,401)	(104,892)
Other Comprehensive Income (Loss), Tax		160,661	645,081
Accumulated Translation Adjustment [Member]
Accumulated other comprehensive income		(104,456)	(121,709)	(172,541)	(181,151)
Other Comprehensive Income (Loss), before Reclassifications, before Tax		23,117	68,271
Reclassification from Accumulated Other Comprehensive Income, Current Period, before Tax		0	0
Other Comprehensive Income (Loss), Tax		44,968	(8,829)
Accumulated Net Unrealized Investment Gain (Loss) [Member]
Accumulated other comprehensive income	[1]	1,770,903	2,381,473	1,355,033	935,697
Other Comprehensive Income (Loss), before Reclassifications, before Tax	[1]	671,564	2,191,823
Reclassification from Accumulated Other Comprehensive Income, Current Period, before Tax	[1]	(51,407)	(109,145)
Other Comprehensive Income (Loss), Tax	[1]	204,287	636,902
Accumulated Defined Benefit Plans Adjustment [Member]
Accumulated other comprehensive income		(40,690)	(47,438)	$ (43,163)	$ (46,262)
Other Comprehensive Income (Loss), before Reclassifications, before Tax		(191)	(6,079)
Reclassification from Accumulated Other Comprehensive Income, Current Period, before Tax		4,006	4,253
Other Comprehensive Income (Loss), Tax		$ 1,342	$ (650)

[1] Includes cash flow hedges of $347 and $(2,496) as of September 30, 2017 and December 31, 2016, respectively, and $(40,597) and $(29,397) as of September 30, 2016 and December 31, 2015, respectively. See Note 5 - “Derivative Instruments” for additional information on cash flow hedges.