Certain Long-Duration Contracts with Guarantees (Variable Annuity, Variable Life, Variable Universal Life and Universal Life Contracts) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Annuity Contracts | Return of net deposits | In the Event of Death
Net Amount at Risk by Product and Guarantee [Line Items]
Account value	$ 94,605,484	$ 85,056,405
Net amount at risk	$ 23,143	$ 178,806
Average attained age of contractholders	65 years	64 years
Annuity Contracts | Minimum return or contract value | In the Event of Death
Net Amount at Risk by Product and Guarantee [Line Items]
Account value	$ 21,331,358	$ 20,091,528
Net amount at risk	$ 1,309,080	$ 2,039,249
Average attained age of contractholders	69 years	68 years
Annuity Contracts | Minimum return or contract value | At Annuitization / Accumulation
Net Amount at Risk by Product and Guarantee [Line Items]
Account value	$ 105,887,964	$ 95,908,923
Net amount at risk	$ 1,729,783	$ 2,875,524
Average attained age of contractholders	66 years	65 years
Average period remaining until earliest expected annuitization	0 years	0 years
Variable Life, Variable Universal Life and Universal Life Contracts | No-lapse guarantees | In the Event of Death
Net Amount at Risk by Product and Guarantee [Line Items]
Separate account value	$ 3,496,913	$ 3,125,804
General account value	7,209,522	6,234,678
Net amount at risk	$ 135,077,843	$ 119,838,053
Average attained age of contractholders	55 years	55 years