Investments - Narrative (Details) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Investment Holdings [Line Items]
Investment Grade Rate	92.00%
Fair value of issued securities	$ 7,630,634	$ 7,456,708
Federal Home Loan Bank Funding Agreements	575,000	575,000
Federal Home Loans Bank Of Chicago
Investment Holdings [Line Items]
Fair value of issued securities	620,558	620,489
Governmental Agencies as Required by Law in Various States
Investment Holdings [Line Items]
Fair value of issued securities	$ 18,133	$ 18,119