Consolidated Balance Sheets (Parenthetical) (USD $) In Thousands, except Share data, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012
Statement Of Financial Position [Abstract]
Securities available for sale, amortized cost	$ 2,804,411	$ 1,986,882
Held to maturity securities	1,848,886	1,710,465
Property and equipment, accumulated depreciation	$ 158,423	$ 160,592
Common stock, par value per share	$ 3.33	$ 3.33
Common stock, shares authorized	350,000,000	350,000,000
Common stock, shares issued	84,881,970	84,881,970
Common stock, shares outstanding	84,847,796	84,847,796