Other Borrowings - Narrative (Details) - USD ($) |
Dec. 31, 2017 |
Dec. 31, 2016 |
|---|---|---|
| Debt Disclosure [Abstract] | ||
| Maximum borrowing capacity, lines of credit | $ 58,000,000 | $ 58,000,000 |
| Long-term line of credit | 0 | 0 |
| Unused borrowing capacity on established lines with FHLB | 1,000,000,000 | 1,000,000,000 |
| Unencumbered residential and commercial real estate loan | 399,200,000 | |
| Overnight FHLB advances | 587,742,000 | 503,815,000 |
| Term advances from FHLB | $ 475,000,000 | $ 365,000,000 |
| Weighted average interest rate (percent) | 1.53% | 1.02% |
| Term advances mature within one year | $ 200,000,000 | |
| Term advances maturing after one year | 275,000,000 | |
| Term advances mature in 2019 | 240,000,000 | |
| Term advances mature in 2020 | 35,000,000 | |
| Callable FHLB borrowings | 0 | |
| Term borrowing with a bank | $ 9,000,000 | $ 16,000,000 |