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Investment Securities (Municipal Bonds Narrative) (Details 6)
$ in Millions
6 Months Ended
Jun. 30, 2018
USD ($)
Debt Securities, Available-for-sale [Line Items]  
Municipal Debt Securities Issued by One Single State or Political Subdivision as Percentage of Total Fair Value of Entire Municipal Debt Securities Threshold 10.00%
HAWAII | Debt Securities Issued by States and Political Subdivisions  
Debt Securities, Available-for-sale [Line Items]  
Debt Securities Issued by State and Political Subdivision, State of Hawaii $ 478.1
Municipal Debt Securities | Geographic Concentration Risk | HAWAII | Debt Securities Issued by States and Political Subdivisions  
Debt Securities, Available-for-sale [Line Items]  
Concentration risk, percentage 57.00%
Hawaiian Municipal Debt Securities | Investment Concentration Risk | HAWAII | General Obligation Bond  
Debt Securities, Available-for-sale [Line Items]  
Concentration risk, percentage 78.00%
Moody's, Aa2 Rating (or better) | Hawaiian Municipal Debt Securities | Investment Concentration Risk | HAWAII | Debt Securities Issued by States and Political Subdivisions  
Debt Securities, Available-for-sale [Line Items]  
Concentration risk, percentage 94.00%