Consolidated Balance Sheet Statement Parenthetical - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Investment securities available-for-sale at amortized cost	$ 1,182,427	$ 1,283,161
Investment securities held-to-maturity at fair value	$ 73,781	$ 92,604
Preferred Stock, Par Value Per Share	$ 1.00	$ 1.00
Preferred Stock, Shares Authorized	2,000,000	2,000,000
Common Stock, Par Value Per Share	$ 1.00	$ 1.00
Common Stock, Shares Authorized	170,000,000	170,000,000
Common Stock, Shares, Issued	84,208,538	83,731,769
Common Stock, Shares, Outstanding	84,208,538	83,731,769
Series D Preferred Stock [Member]
Preferred Stock, Par Value Per Share	$ 1.00
Preferred Stock, Shares Issued	50,000	50,000
Preferred Stock, Shares Outstanding	50,000	50,000
Preferred Stock, Liquidation Preference Per Share	$ 1,000	$ 1,000