Investments - Narrative (Details) $ in Thousands	Mar. 31, 2018 USD ($) positions	Dec. 31, 2017 USD ($)
Schedule of Available-for-sale Securities [Line Items]
Percentage of Fixed Income Investment portfolio invested in below investment grade or not rated	3.50%	3.30%
Percentage of Gross Unrealized losses in Fixed income securities portfolio related to securities below investment grade or not rated	3.50%	4.20%
Pledged Assets Separately Reported, Other Debt Securities Available-for-sale or Held-for-investment	$ 17,900,000	$ 18,800,000
Available for sale securities, Gross unrealized losses	399,667	199,250
Excluding Life Funds Withheld Assets
Schedule of Available-for-sale Securities [Line Items]
Available for sale securities, Gross unrealized losses	$ 399,667	$ 199,250
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions | positions	4,061
Available-for-sale Securities, Qualitative Disclosure, Number of Positions | positions	7,851