Summary of Investments - SOI Securities Lending (Details) - USD ($) $ in Thousands	Jun. 30, 2017	Dec. 31, 2016
Securities Financing Transaction [Line Items]
Payable under securities lending agreements	$ 77,614	$ 0
U.S. government direct obligations and U.S. agencies
Securities Financing Transaction [Line Items]
Payable under securities lending agreements	1,541	0
Corporate debt securities
Securities Financing Transaction [Line Items]
Payable under securities lending agreements	$ 76,073	$ 0