Investments - Narrative (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Investment Holdings [Line Items]
Investment Grade Rate	95.30%
Fair value of issued securities	$ 7,634,682	$ 7,724,075
Federal Home Loan Bank Funding Agreements	625,000	625,000
Federal Home Loans Bank Of Chicago
Investment Holdings [Line Items]
Fair value of issued securities	686,387	686,790
Governmental Agencies as Required by Law in Various States
Investment Holdings [Line Items]
Fair value of issued securities	$ 17,848	$ 17,985