Consolidated Statements of Financial Position (Parenthetical) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Statement of Financial Position [Abstract]
Held-to-maturity, fair value	$ 663,621	$ 662,906
Loan servicing rights, at fair value	$ 68,837	$ 63,841
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
Servicing Asset at Fair Value, Amount	$ 68,837	$ 63,841
Common stock, shares authorized (in shares)	135,000,000	135,000,000
Common stock, shares issued (in shares)	71,350,127	71,207,114
Common stock, shares outstanding (in shares)	71,350,127	71,207,114