Summary of Investments - SOI Securities Lending (Details) - USD ($) $ in Thousands	Mar. 31, 2017	Dec. 31, 2016
Securities Financing Transaction [Line Items]
Payable under securities lending agreements	$ 94,531	$ 0
U.S. government direct obligations and U.S. agencies
Securities Financing Transaction [Line Items]
Payable under securities lending agreements	1,025	0
Corporate debt securities
Securities Financing Transaction [Line Items]
Payable under securities lending agreements	$ 93,506	$ 0