Consolidated Balance Sheets (Parenthetical) - USD ($) $ in Thousands	Sep. 30, 2016	Dec. 31, 2015
Consolidated Balance Sheets [Abstract]
Securities available for sale, amortized cost	$ 2,370,621	$ 2,086,745
Securities held to maturity, fair value	2,482,540	2,375,851
Property and equipment, accumulated depreciation	$ 227,075	$ 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	77,570,515	77,496,429
Treasury shares at cost, shares	9,246,656	9,319,082