Investments (Details Textual) - USD ($) $ in Thousands	Mar. 31, 2016	Dec. 31, 2015
Investments (Textual) [Abstract]
Fair value of issued securities	$ 7,216,502	$ 7,091,340
Investments (Additional Textual) [Abstract]
Federal Home Loan Bank Funding Agreements	$ 575,000	575,000
Investment Grade Rate	66.00%
Federal Home Loans Bank Of Chicago [Member]
Investments (Textual) [Abstract]
Fair value of issued securities	$ 621,373	621,077
Governmental Agencies as Required by Law in Various States [Member]
Investments (Textual) [Abstract]
Fair value of issued securities	18,392	18,312
Federal National Mortgage Association Certificates and Obligations (FNMA) [Member]
Investments (Textual) [Abstract]
Fair value of issued securities	234,668	231,294
Federal Home Loan Mortgage Corporation Certificates and Obligations (FHLMC) [Member]
Investments (Textual) [Abstract]
Fair value of issued securities	342,877	363,957
Government National Mortgage Association Certificates and Obligations (GNMA) [Member]
Investments (Textual) [Abstract]
Fair value of issued securities	$ 126,252	$ 130,940