Consolidated Statements of Financial Position (Parenthetical) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Statement of Financial Position [Abstract]
Held-to-maturity, fair value	$ 662,906	$ 608,531
Loan servicing rights, fair value	$ 63,841	$ 48,085
Preferred stock, no par value (in dollars per share)	$ 0	$ 0
Preferred stock, shares authorized	2,000,000	2,000,000
Preferred stock, shares issued	0	0
Common stock, par value (in dollars per share)	$ 1	$ 1
Common stock, shares authorized	135,000,000	100,000,000
Common stock, shares issued	71,207,114	70,599,133
Common stock, shares outstanding	71,207,114	70,599,133