Investments (Details Textual) - USD ($) $ in Thousands	Sep. 30, 2015	Dec. 31, 2014
Investments (Textual) [Abstract]
Fair value of issued securities	$ 7,078,742	$ 6,893,090
Investments (Additional Textual) [Abstract]
Federal Home Loan Bank Funding Agreements	$ 500,000	500,000
Investment Grade Rate	59.00%
Federal Home Loans Bank Of Chicago [Member]
Investments (Textual) [Abstract]
Fair value of issued securities	$ 537,400	539,235
Governmental Agencies as Required by Law in Various States [Member]
Investments (Textual) [Abstract]
Fair value of issued securities	18,423	18,361
Federal National Mortgage Association Certificates and Obligations (FNMA) [Member]
Investments (Textual) [Abstract]
Fair value of issued securities	254,810	302,222
Federal Home Loan Mortgage Corporation Certificates and Obligations (FHLMC) [Member]
Investments (Textual) [Abstract]
Fair value of issued securities	406,845	432,432
Government National Mortgage Association Certificates and Obligations (GNMA) [Member]
Investments (Textual) [Abstract]
Fair value of issued securities	$ 137,291	$ 137,867