Investments Investments in Any One Issuer Exceeding Ten Percent of Equity (Details) - USD ($) $ in Thousands	12 Months Ended
Dec. 31, 2017	Dec. 31, 2016
Concentration Risk [Line Items]
Available-for-sale Securities, Amortized Cost Basis	$ 1,182,427	$ 1,283,161
Available-for-sale Securities	1,170,328	$ 1,264,132
Credit Concentration Risk [Member]
Concentration Risk [Line Items]
Available-for-sale Securities, Amortized Cost Basis	876,373
Available-for-sale Securities	$ 861,205
Stockholders' Equity, Total [Member]
Concentration Risk [Line Items]
Concentration Risk, Percentage	10.00%
Government National Mortgage Association \| Credit Concentration Risk [Member]
Concentration Risk [Line Items]
Available-for-sale Securities, Amortized Cost Basis	$ 104,188
Available-for-sale Securities	101,941
Federal Home Loan Mortgage Corporation \| Credit Concentration Risk [Member]
Concentration Risk [Line Items]
Available-for-sale Securities, Amortized Cost Basis	349,926
Available-for-sale Securities	342,374
Federal Home Loan Bank \| Credit Concentration Risk [Member]
Concentration Risk [Line Items]
Available-for-sale Securities, Amortized Cost Basis	163,151
Available-for-sale Securities	161,747
Federal National Mortgage Association \| Credit Concentration Risk [Member]
Concentration Risk [Line Items]
Available-for-sale Securities, Amortized Cost Basis	259,108
Available-for-sale Securities	$ 255,143

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