CONSOLIDATED BALANCE SHEETS CONSOLIDATED BALANCE SHEETS (Parenthetical) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Statement of Financial Position [Abstract]
Available-for-sale Securities, Amortized Cost Basis	$ 1,370,844	$ 1,348,227
Held-to-maturity Securities, Fair Value	$ 623,967	$ 653,101
Common Stock, No Par Value	$ 0	$ 0
Common Stock, Shares Authorized	160,000,000	160,000,000
Common Stock, Shares, Issued	68,730,731	68,730,731
Treasury Stock, Shares	6,516,908	6,661,644