Minimum Guarantees - Fair Value of Equity Securities Supporting Separate Accounts (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Schedule of Fair Value of Separate Accounts by Major Category of Investment [Line Items]
Fair value of assets supporting separate accounts	$ 686,980	$ 468,582
GMDB
Schedule of Fair Value of Separate Accounts by Major Category of Investment [Line Items]
Fair value of assets supporting separate accounts	64,975	55,605
GLWB
Schedule of Fair Value of Separate Accounts by Major Category of Investment [Line Items]
Fair value of assets supporting separate accounts	$ 622,005	$ 412,977