Fair Value Measurements - Estimated Fair Values of Financial Instruments (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Financial assets:
Interest receivable	$ 13,920	$ 13,069
Mortgage servicing rights, net	93,160	76,107	$ 33,081
Carrying Amount
Financial assets:
Cash and cash equivalents	73,700	119,751
Investment securities	597,347	543,992
Federal Home Loan Bank Stock	11,810	11,412
Loans, net	3,220,257	3,142,870
Loans held for sale	414,518	526,185
Interest receivable	13,920	13,069
Mortgage servicing rights, net	93,160	76,107
Derivatives assets	14,904	9,690
Financial liabilities:
Deposits, Without stated maturities	3,059,142	2,976,066
Deposits, With stated maturities	707,009	688,329
Securities sold under agreement to repurchase	14,724	14,293
Short term borrowings	138,707	190,000
Interest payable	1,857	1,504
Long-term debt	139,586	143,302
Derivatives liabilities	3,655	1,699
Fair Value
Financial assets:
Cash and cash equivalents	73,700	119,751
Investment securities	597,347	543,992
Loans, net	3,213,659	3,141,400
Loans held for sale	414,518	526,185
Interest receivable	13,920	13,069
Mortgage servicing rights, net	93,160	76,107
Derivatives assets	14,904	9,690
Financial liabilities:
Deposits, Without stated maturities	3,059,142	2,976,066
Deposits, With stated maturities	706,585	682,403
Securities sold under agreement to repurchase	14,724	14,293
Short term borrowings	138,707	190,000
Interest payable	1,857	1,504
Long-term debt	138,456	149,135
Derivatives liabilities	3,655	1,699
Fair Value | Fair Value Level 1
Financial assets:
Cash and cash equivalents	73,700	119,751
Financial liabilities:
Deposits, Without stated maturities	3,059,142	2,976,066
Securities sold under agreement to repurchase	14,724	14,293
Short term borrowings	138,707	190,000
Interest payable	526	575
Fair Value | Fair Value Level 2
Financial assets:
Investment securities	597,347	540,388
Loans, net		3,064,373
Loans held for sale	414,518	526,185
Interest receivable	13,920	13,069
Derivatives assets	14,904	9,690
Financial liabilities:
Deposits, With stated maturities	706,585	682,403
Interest payable	1,331	929
Long-term debt	138,456	149,135
Derivatives liabilities	3,655	1,699
Fair Value | Fair Value Level 3
Financial assets:
Investment securities		3,604
Loans, net	3,213,659	77,027
Mortgage servicing rights, net	$ 93,160	$ 76,107