Consolidated Balance Sheets (Parenthetical) - USD ($) $ in Thousands	Jun. 30, 2015	Dec. 31, 2014
Statement of Financial Position [Abstract]
Securities available for sale, amortized cost	$ 2,136,856	$ 1,631,761
Held to maturity securities	2,310,103	2,186,340
Property and equipment, accumulated depreciation	$ 198,595	$ 193,527
Common stock, par value per share	$ 3.33	$ 3.33
Common stock, shares authorized	350,000,000	350,000,000
Common stock, shares outstanding	78,093,729	80,426,485
Treasury shares, shares	8,721,782	7,053,028