Equity (Balance of and Changes in Each Component of AOCI) (Details) - USD ($) $ in Thousands		3 Months Ended
		Mar. 31, 2019	Mar. 31, 2018
Balance, Beginning of Period		$ 636,763	$ 2,063,631
Other Comprehensive Income (Loss), before Reclassifications, before Tax		1,453,733	(823,915)
Reclassification from Accumulated Other Comprehensive Income, Current Period, before Tax		(13,408)	22,620
Other Comprehensive Income (Loss), Tax		311,123	(166,060)
Balance, End of Period		1,765,965	1,428,396
Accumulated Translation Adjustment [Member]
Balance, Beginning of Period		(168,698)	(86,350)
Other Comprehensive Income (Loss), before Reclassifications, before Tax		18,462	5,696
Reclassification from Accumulated Other Comprehensive Income, Current Period, before Tax			0
Other Comprehensive Income (Loss), Tax		2,924	(6,856)
Balance, End of Period		(147,312)	(87,510)
Accumulated Net Unrealized Investment Gain (Loss) [Member]
Balance, Beginning of Period	[1]	856,159	2,200,661
Other Comprehensive Income (Loss), before Reclassifications, before Tax	[1]	1,437,058	(827,701)
Reclassification from Accumulated Other Comprehensive Income, Current Period, before Tax	[1]	(14,694)	21,274
Other Comprehensive Income (Loss), Tax	[1]	314,153	(172,823)
Balance, End of Period	[1]	1,964,370	1,567,057
Accumulated Defined Benefit Plans Adjustment [Member]
Balance, Beginning of Period		(50,698)	(50,680)
Other Comprehensive Income (Loss), before Reclassifications, before Tax		(1,787)	(1,910)
Reclassification from Accumulated Other Comprehensive Income, Current Period, before Tax		1,286	1,346
Other Comprehensive Income (Loss), Tax		(106)	(93)
Balance, End of Period		$ (51,093)	$ (51,151)

[1] Includes cash flow hedges of $(1,902) and $8,788 as of March 31, 2019 and December 31, 2018, respectively, and $20,662 and $2,619 as of March 31, 2018 and December 31, 2017, respectively. See Note 5 - “Derivative Instruments” for additional information on cash flow hedges.