Consolidated Balance Sheets (Parenthetical) - USD ($) $ in Thousands	Mar. 31, 2016	Dec. 31, 2015
Consolidated Balance Sheets [Abstract]
Securities available for sale, amortized cost	$ 2,116,271	$ 2,086,745
Securities held to maturity, fair value	2,566,799	2,375,851
Property and equipment, accumulated depreciation	$ 216,418	$ 209,763
Common stock, par value per share	$ 3.33	$ 3.33
Common stock, shares authorized	350,000,000	350,000,000
Common stock, shares outstanding	77,507,960	77,496,429
Treasury shares at cost, shares	9,307,551	9,319,082