Investments - Narrative (Details) $ in Thousands	Dec. 31, 2017 USD ($) positions	Dec. 31, 2016 USD ($)
Marketable Securities [Line Items]
Percentage of fixed income portfolio invested in securities below investment grade	3.30%	2.50%
Percentage of gross unrealized losses relating to securities below investment grade	4.20%	4.10%
Pledged assets, amount	$ 18,800,000	$ 17,100,000
Gross unrealized losses	(199,250)	(370,017)
Exclude Life Funds Withheld Assets
Marketable Securities [Line Items]
Gross unrealized losses	$ (199,250)	$ (370,017)
Number of positions in a loss position (number of positions) | positions	2,950
Total number of positions | positions	7,822