Consolidated Statements of Financial Position (Parenthetical) - USD ($) $ in Thousands	Jun. 30, 2017	Dec. 31, 2016
Statement of Financial Position [Abstract]
Held-to-maturity, fair value	$ 640,043	$ 608,531
Loan servicing rights, fair value	$ 64,522	$ 48,085
Preferred stock, no par value (dollars per share)
Preferred stock, shares authorized (in shares)	2,000,000	2,000,000
Preferred stock, shares issued (in shares)	0	0
Common stock, par value (dollars per share)	$ 1	$ 1
Common stock, shares authorized (in shares)	135,000,000	100,000,000
Common stock, shares issued (in shares)	71,131,042	70,599,133
Common stock, shares outstanding (in shares)	71,131,042	70,599,133