Investments (Schedule of Credit Quality of Fixed Maturity Portfolio by NAIC Designation) (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Investment Holdings [Line Items]
Amortized cost	$ 45,765,555	$ 43,193,800
Fair value	45,893,130	45,449,449
NAIC, Class 1 Designation
Investment Holdings [Line Items]
Amortized cost	27,186,979	26,669,427
Fair value	27,478,961	28,274,379
NAIC, Class 2 Designation
Investment Holdings [Line Items]
Amortized cost	16,972,448	15,198,551
Fair value	16,868,800	15,869,219
NAIC, Class 3 Designation
Investment Holdings [Line Items]
Amortized cost	1,406,867	1,161,737
Fair value	1,365,147	1,157,420
NAIC, Class 4 Designation
Investment Holdings [Line Items]
Amortized cost	180,233	134,838
Fair value	156,449	117,542
NAIC, Class 5 Designation
Investment Holdings [Line Items]
Amortized cost	11,200	17,015
Fair value	16,898	20,927
NAIC, Class 6 Designation
Investment Holdings [Line Items]
Amortized cost	7,828	12,232
Fair value	$ 6,875	$ 9,962