Investments Investments in Any One Issuer Exceeding Ten Percent of Equity (Details) - USD ($) $ in Thousands	12 Months Ended
Dec. 31, 2016	Dec. 31, 2015
Concentration Risk [Line Items]
Available-for-sale Securities, Amortized Cost Basis	$ 1,283,161	$ 1,084,105
Available-for-sale Securities	1,264,132	$ 1,084,510
Credit Concentration Risk [Member]
Concentration Risk [Line Items]
Available-for-sale Securities, Amortized Cost Basis	676,805
Available-for-sale Securities	$ 669,623
Stockholders' Equity, Total [Member]
Concentration Risk [Line Items]
Concentration Risk, Percentage	10.00%
Government National Mortgage Association Certificates and Obligations (GNMA) [Member] \| Credit Concentration Risk [Member]
Concentration Risk [Line Items]
Available-for-sale Securities, Amortized Cost Basis	$ 80,984
Available-for-sale Securities	79,550
Federal Home Loan Mortgage Corporation Certificates and Obligations (FHLMC) [Member] \| Credit Concentration Risk [Member]
Concentration Risk [Line Items]
Available-for-sale Securities, Amortized Cost Basis	144,305
Available-for-sale Securities	143,638
Federal Home Loan Bank Certificates and Obligations (FHLB) [Member] \| Credit Concentration Risk [Member]
Concentration Risk [Line Items]
Available-for-sale Securities, Amortized Cost Basis	119,811
Available-for-sale Securities	119,141
Federal National Mortgage Association Certificates and Obligations (FNMA) [Member] \| Credit Concentration Risk [Member]
Concentration Risk [Line Items]
Available-for-sale Securities, Amortized Cost Basis	331,705
Available-for-sale Securities	$ 327,294

Amount of investment in debt and equity securities categorized neither as held-to-maturity nor trading.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
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+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
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