Investments (Schedule of Fixed Maturity Securities) (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Debt Securities, Available-for-sale [Line Items]
Fixed maturity securities, available for sale, amortized cost	$ 48,170,556	$ 46,131,190
Fixed maturity securities, available for sale, gross unrealized gains	3,990,859	1,143,624
Fixed maturity securities, available for sale, gross unrealized losses	(307,031)	(1,351,087)
Fixed maturity securities, available for sale, fair value	51,854,384	45,923,727
United States Government Full Faith and Credit
Debt Securities, Available-for-sale [Line Items]
Fixed maturity securities, available for sale, amortized cost	11,832	11,872
Fixed maturity securities, available for sale, gross unrealized gains	511	102
Fixed maturity securities, available for sale, gross unrealized losses	0	(322)
Fixed maturity securities, available for sale, fair value	12,343	11,652
United States Government Sponsored Agencies
Debt Securities, Available-for-sale [Line Items]
Fixed maturity securities, available for sale, amortized cost	663,662	1,208,468
Fixed maturity securities, available for sale, gross unrealized gains	33,420	13,095
Fixed maturity securities, available for sale, gross unrealized losses	(2,052)	(83,034)
Fixed maturity securities, available for sale, fair value	695,030	1,138,529
United States Municipalities, States and Territories
Debt Securities, Available-for-sale [Line Items]
Fixed maturity securities, available for sale, amortized cost	4,053,836	3,880,703
Fixed maturity securities, available for sale, gross unrealized gains	464,539	261,222
Fixed maturity securities, available for sale, gross unrealized losses	(714)	(15,658)
Fixed maturity securities, available for sale, fair value	4,517,661	4,126,267
Foreign Government Obligations
Debt Securities, Available-for-sale [Line Items]
Fixed maturity securities, available for sale, amortized cost	186,979	226,860
Fixed maturity securities, available for sale, gross unrealized gains	20,441	7,573
Fixed maturity securities, available for sale, gross unrealized losses	0	(4,159)
Fixed maturity securities, available for sale, fair value	207,420	230,274
Corporate Securities
Debt Securities, Available-for-sale [Line Items]
Fixed maturity securities, available for sale, amortized cost	30,042,589	28,483,138
Fixed maturity securities, available for sale, gross unrealized gains	2,959,486	727,105
Fixed maturity securities, available for sale, gross unrealized losses	(104,948)	(838,729)
Fixed maturity securities, available for sale, fair value	32,897,127	28,371,514
Residential Mortgage Backed Securities
Debt Securities, Available-for-sale [Line Items]
Fixed maturity securities, available for sale, amortized cost	1,434,116	1,134,623
Fixed maturity securities, available for sale, gross unrealized gains	124,648	71,661
Fixed maturity securities, available for sale, gross unrealized losses	(1,608)	(4,125)
Fixed maturity securities, available for sale, fair value	1,557,156	1,202,159
Commercial Mortgage Backed Securities
Debt Securities, Available-for-sale [Line Items]
Fixed maturity securities, available for sale, amortized cost	5,564,871	5,492,271
Fixed maturity securities, available for sale, gross unrealized gains	279,881	21,558
Fixed maturity securities, available for sale, gross unrealized losses	(5,242)	(134,826)
Fixed maturity securities, available for sale, fair value	5,839,510	5,379,003
Other Asset Backed Securities
Debt Securities, Available-for-sale [Line Items]
Fixed maturity securities, available for sale, amortized cost	6,212,671	5,693,255
Fixed maturity securities, available for sale, gross unrealized gains	107,933	41,308
Fixed maturity securities, available for sale, gross unrealized losses	(192,467)	(270,234)
Fixed maturity securities, available for sale, fair value	$ 6,128,137	$ 5,464,329

Amount, before tax, of unrealized gain in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
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-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
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Amount, before tax, of unrealized loss in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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