Fair Value Measurements - Estimated Fair Values of Financial Instruments (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Financial assets:
Available-for-sale securities, at fair value	$ 543,992	$ 582,183
Interest receivable	13,069	7,241
Mortgage servicing rights, net		33,081	$ 29,711	$ 6,032
Derivatives assets	9,690	19,745
Financial liabilities:
Derivatives liabilities	1,699	586
Carrying Amount
Financial assets:
Cash and cash equivalents	119,751	136,327
Available-for-sale securities, at fair value	543,992	582,183
Federal Home Loan Bank Stock	11,412	7,743
Loans, net	3,142,870	1,827,037
Loans held for sale	526,185	507,442
Interest receivable	13,069	7,241
Mortgage servicing rights, net	76,107	32,070
Derivatives assets	9,690	19,745
Financial liabilities:
Deposits, Without stated maturities	2,976,066	2,280,531
Deposits, With stated maturities	688,329	391,031
Securities sold under agreement to repurchase	14,293	21,561
Short term borrowings	190,000	150,000
Interest payable	1,504	632
Long-term debt	143,302	44,892
Derivatives liabilities	1,699	586
Fair Value
Financial assets:
Cash and cash equivalents	119,751	136,327
Available-for-sale securities, at fair value	543,992	582,183
Federal Home Loan Bank Stock	11,412	7,743
Loans, net	3,141,400	1,823,335
Loans held for sale	526,185	507,442
Interest receivable	13,069	7,241
Mortgage servicing rights, net	76,107	33,081
Derivatives assets	9,690	19,745
Financial liabilities:
Deposits, Without stated maturities	2,976,066	2,280,531
Deposits, With stated maturities	682,403	390,484
Securities sold under agreement to repurchase	14,293	21,561
Short term borrowings	190,000	150,000
Interest payable	1,504	632
Long-term debt	149,135	47,377
Derivatives liabilities	1,699	586
Fair Value | Fair Value Level 1
Financial assets:
Cash and cash equivalents	119,751	136,327
Financial liabilities:
Deposits, Without stated maturities	2,976,066	2,280,531
Securities sold under agreement to repurchase	14,293	21,561
Short term borrowings	190,000	150,000
Interest payable	575	249
Fair Value | Fair Value Level 2
Financial assets:
Available-for-sale securities, at fair value	540,388	577,634
Loans, net	3,064,373	1,822,054
Loans held for sale	526,185	507,442
Interest receivable	13,069	7,241
Derivatives assets	9,690	19,745
Financial liabilities:
Deposits, With stated maturities	682,403	390,484
Interest payable	929	383
Long-term debt	149,135	47,377
Derivatives liabilities	1,699	586
Fair Value | Fair Value Level 3
Financial assets:
Available-for-sale securities, at fair value	3,604	4,549
Federal Home Loan Bank Stock	11,412	7,743
Loans, net	77,027	1,281
Mortgage servicing rights, net	$ 76,107	$ 33,081