Investments (Details Textual) - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014
Investments (Textual) [Abstract]
Fair value of issued securities	$ 7,091,340	$ 6,893,090
Investments (Additional Textual) [Abstract]
Federal Home Loan Bank Funding Agreements	$ 575,000	500,000
Investment Grade Rate	75.00%
Federal Home Loans Bank Of Chicago [Member]
Investments (Textual) [Abstract]
Fair value of issued securities	$ 621,077	539,235
Governmental Agencies as Required by Law in Various States [Member]
Investments (Textual) [Abstract]
Fair value of issued securities	18,312	18,361
Federal National Mortgage Association Certificates and Obligations (FNMA) [Member]
Investments (Textual) [Abstract]
Fair value of issued securities	231,294	302,222
Federal Home Loan Mortgage Corporation Certificates and Obligations (FHLMC) [Member]
Investments (Textual) [Abstract]
Fair value of issued securities	363,957	432,432
Government National Mortgage Association Certificates and Obligations (GNMA) [Member]
Investments (Textual) [Abstract]
Fair value of issued securities	$ 130,940	$ 137,867