Summary of Investments - Schedule of cash collateral liability (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Securities Financing Transaction [Line Items]
Cash collateral liability by class of loaned security	$ 66,784	$ 0
U.S. government direct obligations and U.S. agencies
Securities Financing Transaction [Line Items]
Cash collateral liability by class of loaned security	7,700	0
Corporate debt securities
Securities Financing Transaction [Line Items]
Cash collateral liability by class of loaned security	$ 59,084	$ 0