Consolidated Balance Sheets (Unaudited) (Parenthetical) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Cost of available for sale securities	$ 2,124,232	$ 1,893,947
Fair value of held to maturity securities	$ 270,386	$ 274,799
Common stock, par value (in dollars per share)	$ 1	$ 1
Common stock, shares authorized (shares)	40,000,000	30,000,000
Common stock, shares issued (shares)	29,946,069	26,119,929
Treasury stock (shares)	0	0
Preferred Stock
Preferred stock, par value (in dollars per share)	$ 1	$ 1
Preferred stock, shares authorized (shares)	17,604	17,604
Preferred stock, shares issued (shares)	0	0
Preferred stock, shares outstanding (shares)	0	0
Series A Junior Participating Preferred Stock
Preferred stock, par value (in dollars per share)	$ 1	$ 1
Preferred stock, shares authorized (shares)	16,000	16,000
Preferred stock, shares issued (shares)	0	0
Preferred stock, shares outstanding (shares)	0	0
Series C Senior Non-Cumulative Perpetual Preferred Stock
Preferred stock, par value (in dollars per share)	$ 1	$ 1
Preferred stock, shares authorized (shares)	81,698	81,698
Preferred stock, shares issued (shares)	0	0
Preferred stock, shares outstanding (shares)	0	0
Series D Senior Non-Cumulative Perpetual Preferred Stock
Preferred stock, par value (in dollars per share)	$ 1	$ 1
Preferred stock, shares authorized (shares)	3,000	3,000
Preferred stock, shares issued (shares)	745	1,078
Preferred stock, shares outstanding (shares)	745	1,078