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Debt - Narrative (Details) - USD ($) |  |  | 12 Months Ended
--- | --- | --- | ---
Dec. 07, 2017 | Oct. 10, 2017 | Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
Debt Instrument [Line Items] |  |  |  |  |
FHLB borrowings | $ 25,000,000 | $ 50,000,000 | $ 50,000,000 | $ 0 | $ 0
FHLB advances interest rate |  |  | 1.57% |  |
Current Unsecured Bank Credit Facility |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |
Maximum borrowing capacity |  |  | $ 150,000,000 |  |
Basis spread on variable rate |  |  | 1.15% |  |
Variable commitment fee |  |  | 0.15% |  |
4.50% Senior Notes |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |
Stated rate |  |  | 4.50% |  |
Principal amount |  |  | $ 250,000,000 |  |
Discount percent on issuance |  |  | 0.265% |  |
Effective interest percentage |  |  | 4.533% |  |
October 5, 2022 |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |
FHLB Advances |  |  | $ 25,000,000 |  |
December 2, 2022 |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |
FHLB Advances |  |  | 25,000,000 |  |
Long-term debt |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |
FHLB Advances |  |  | $ 50,000,000 |  |

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