Consolidated Statements of Financial Position (Parenthetical) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Statement of Financial Position [Abstract]
Held-to-maturity, fair value	$ 655,959	$ 608,531
Loan servicing rights ($62,195 and $48,085 measured at fair value, respectively)	$ 62,195	$ 58,315
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
Servicing Asset at Fair Value, Amount	$ 62,195	$ 48,085
Common stock, shares authorized (in shares)	135,000,000	100,000,000
Common stock, shares issued (in shares)	71,151,697	70,599,133
Common stock, shares outstanding (in shares)	71,151,697	70,599,133