Consolidated Statements of Financial Position (Parenthetical) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Statement of Financial Position [Abstract]
Held-to-maturity, fair value	$ 627,615	$ 618,672
Loan servicing rights, at fair value	$ 64,701	$ 71,013
Preferred stock, no par value (dollars per share)	$ 0	$ 0
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	135,000,000
Common stock, shares issued (in shares)	71,550,673	71,460,119
Common stock, shares outstanding (in shares)	71,550,673	71,460,119