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Investments (Fixed Maturity Securities Pledged And Received As Collateral) (Details) - USD ($)
$ in Thousands
Mar. 31, 2018
Dec. 31, 2017
Schedule of Available-for-sale Securities [Line Items]    
Available-for-sale Securities, Amortized Cost Basis $ 22,890,985 $ 21,966,803
Available-for-sale Securities 23,520,469 23,210,968
Collateral Fixed Maturity Securities [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Available-for-sale Securities, Amortized Cost Basis 72,850 72,542
Available-for-sale Securities 75,384 75,622
Collateral Received Fixed Maturity Securities [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Available-for-sale Securities 614,423 590,417
Reported Value Measurement [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Assets Held-in-trust 16,316,662 15,584,296
Estimate of Fair Value Measurement [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Assets Held-in-trust $ 17,076,547 $ 16,715,281