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