Investments (Schedule of Fixed Maturity Securities) (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Debt Securities, Available-for-sale [Line Items]
Fixed maturity securities, available for sale, amortized cost	$ 46,727,050	$ 46,131,190
Fixed maturity securities, available for sale, gross unrealized gains	1,890,597	1,143,624
Fixed maturity securities, available for sale, gross unrealized losses	(580,540)	(1,351,087)
Fixed maturity securities, available for sale, fair value	48,037,107	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	11,707	11,872
Fixed maturity securities, available for sale, gross unrealized gains	138	102
Fixed maturity securities, available for sale, gross unrealized losses	(144)	(322)
Fixed maturity securities, available for sale, fair value	11,701	11,652
United States Government Sponsored Agencies
Debt Securities, Available-for-sale [Line Items]
Fixed maturity securities, available for sale, amortized cost	1,208,461	1,208,468
Fixed maturity securities, available for sale, gross unrealized gains	18,707	13,095
Fixed maturity securities, available for sale, gross unrealized losses	(39,828)	(83,034)
Fixed maturity securities, available for sale, fair value	1,187,340	1,138,529
United States Municipalities, States and Territories
Debt Securities, Available-for-sale [Line Items]
Fixed maturity securities, available for sale, amortized cost	3,861,080	3,880,703
Fixed maturity securities, available for sale, gross unrealized gains	317,345	261,222
Fixed maturity securities, available for sale, gross unrealized losses	(3,579)	(15,658)
Fixed maturity securities, available for sale, fair value	4,174,846	4,126,267
Foreign Government Obligations
Debt Securities, Available-for-sale [Line Items]
Fixed maturity securities, available for sale, amortized cost	226,233	226,860
Fixed maturity securities, available for sale, gross unrealized gains	10,960	7,573
Fixed maturity securities, available for sale, gross unrealized losses	(1,651)	(4,159)
Fixed maturity securities, available for sale, fair value	235,542	230,274
Corporate Securities
Debt Securities, Available-for-sale [Line Items]
Fixed maturity securities, available for sale, amortized cost	29,025,709	28,483,138
Fixed maturity securities, available for sale, gross unrealized gains	1,312,764	727,105
Fixed maturity securities, available for sale, gross unrealized losses	(324,276)	(838,729)
Fixed maturity securities, available for sale, fair value	30,014,197	28,371,514
Residential Mortgage Backed Securities
Debt Securities, Available-for-sale [Line Items]
Fixed maturity securities, available for sale, amortized cost	1,116,718	1,134,623
Fixed maturity securities, available for sale, gross unrealized gains	86,066	71,661
Fixed maturity securities, available for sale, gross unrealized losses	(1,452)	(4,125)
Fixed maturity securities, available for sale, fair value	1,201,332	1,202,159
Commercial Mortgage Backed Securities
Debt Securities, Available-for-sale [Line Items]
Fixed maturity securities, available for sale, amortized cost	5,493,082	5,492,271
Fixed maturity securities, available for sale, gross unrealized gains	90,317	21,558
Fixed maturity securities, available for sale, gross unrealized losses	(39,887)	(134,826)
Fixed maturity securities, available for sale, fair value	5,543,512	5,379,003
Other Asset Backed Securities
Debt Securities, Available-for-sale [Line Items]
Fixed maturity securities, available for sale, amortized cost	5,784,060	5,693,255
Fixed maturity securities, available for sale, gross unrealized gains	54,300	41,308
Fixed maturity securities, available for sale, gross unrealized losses	(169,723)	(270,234)
Fixed maturity securities, available for sale, fair value	$ 5,668,637	$ 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
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (b)
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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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