Fair Value Measurements - Estimated Fair Values of Financial Instruments (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Mar. 31, 2017	Dec. 31, 2016
Financial assets:
Interest receivable	$ 12,729		$ 13,069
Mortgage servicing rights, net	109,449	$ 93,160	76,107	$ 47,593	$ 33,081
Carrying Amount
Financial assets:
Cash and cash equivalents	104,417		119,751
Investment securities	611,435		543,992
Federal Home Loan Bank Stock	12,641		11,412
Loans, net	3,389,228		3,142,870
Loans held for sale	374,916		526,185
Interest receivable	12,729		13,069
Mortgage servicing rights, net	109,449		76,107
Derivatives assets	15,117		9,690
Financial liabilities:
Deposits, Without stated maturities	3,179,754		2,976,066
Deposits, With stated maturities	730,109		688,329
Securities sold under agreement to repurchase	15,996		14,293
Short term borrowings	187,522		190,000
Interest payable	2,180		1,504
Long-term debt	139,375		143,302
Derivatives liabilities	5,480		1,699
Fair Value
Financial assets:
Cash and cash equivalents	104,417		119,751
Investment securities	611,435		543,992
Loans, net	3,384,932		3,141,400
Loans held for sale	374,916		526,185
Interest receivable	12,729		13,069
Mortgage servicing rights, net	109,449		76,107
Derivatives assets	15,117		9,690
Financial liabilities:
Deposits, Without stated maturities	3,179,754		2,976,066
Deposits, With stated maturities	731,029		682,403
Securities sold under agreement to repurchase	15,996		14,293
Short term borrowings	187,522		190,000
Interest payable	2,180		1,504
Long-term debt	138,578		149,135
Derivatives liabilities	5,480		1,699
Fair Value | Fair Value Level 1
Financial assets:
Cash and cash equivalents	104,417		119,751
Financial liabilities:
Deposits, Without stated maturities	3,179,754		2,976,066
Securities sold under agreement to repurchase	15,996		14,293
Short term borrowings	187,522		190,000
Interest payable	631		575
Fair Value | Fair Value Level 2
Financial assets:
Investment securities	611,435		540,388
Loans, net			3,064,373
Loans held for sale	374,916		526,185
Interest receivable	12,729		13,069
Derivatives assets	15,117		9,690
Financial liabilities:
Deposits, With stated maturities	731,029		682,403
Interest payable	1,549		929
Long-term debt	138,578		149,135
Derivatives liabilities	5,480		1,699
Fair Value | Fair Value Level 3
Financial assets:
Investment securities			3,604
Loans, net	3,384,932		77,027
Mortgage servicing rights, net	$ 109,449		$ 76,107