Investments (Details Textual) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015
Investments (Textual) [Abstract]
Fair value of issued securities	$ 7,456,708	$ 7,091,340
Investments (Additional Textual) [Abstract]
Federal Home Loan Bank Funding Agreements	$ 575,000
Investment Grade Rate	88.00%
Fair Value, Inputs, Level 3 [Member]
Investments (Additional Textual) [Abstract]
Asset Impairment Charges	$ 1,833
Federal Home Loans Bank Of Chicago [Member]
Investments (Textual) [Abstract]
Fair value of issued securities	620,489	621,077
Governmental Agencies as Required by Law in Various States [Member]
Investments (Textual) [Abstract]
Fair value of issued securities	18,119	18,312
Federal National Mortgage Association Certificates and Obligations (FNMA) [Member]
Investments (Textual) [Abstract]
Fair value of issued securities	196,468	231,294
Federal Home Loan Mortgage Corporation Certificates and Obligations (FHLMC) [Member]
Investments (Textual) [Abstract]
Fair value of issued securities	284,050	363,957
Government National Mortgage Association Certificates and Obligations (GNMA) [Member]
Investments (Textual) [Abstract]
Fair value of issued securities	$ 115,627	$ 130,940