Investments (Details Textual) - USD ($) $ in Thousands		Mar. 31, 2017	Dec. 31, 2016
Investments (Textual) [Abstract]
Fair value of issued securities		$ 7,510,750	$ 7,456,708
Investments (Additional Textual) [Abstract]
Federal Home Loan Bank Funding Agreements		$ 575,000	575,000
Investment Grade Rate	[1]	90.00%
Federal Home Loans Bank Of Chicago [Member]
Investments (Textual) [Abstract]
Fair value of issued securities		$ 621,544	620,489
Governmental Agencies as Required by Law in Various States [Member]
Investments (Textual) [Abstract]
Fair value of issued securities		18,089	18,119
Federal National Mortgage Association Certificates and Obligations (FNMA) [Member]
Investments (Textual) [Abstract]
Fair value of issued securities		283,039	272,668
Federal Home Loan Mortgage Corporation Certificates and Obligations (FHLMC) [Member]
Investments (Textual) [Abstract]
Fair value of issued securities		373,017	378,683
Government National Mortgage Association Certificates and Obligations (GNMA) [Member]
Investments (Textual) [Abstract]
Fair value of issued securities		$ 112,065	$ 115,627

[1] Includes nonredeemable (perpetual) preferred stocks, common stocks and closed-end funds.