Consolidated Balance Sheets (Parenthetical) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Statement of Financial Position [Abstract]
Securities held to maturity, fair value	$ 0	$ 362,893
Preferred stock, par value (usd per share)	$ 0.01	$ 0.01
Preferred stock, shares authorized (shares)	5,000,000	5,000,000
Preferred stock, shares issued (shares)	0	0
Preferred stock, shares outstanding (shares)	0	0
Common stock, par value (usd per share)	$ 5	$ 5
Common stock, shares authorized (shares)	150,000,000	150,000,000
Common stock, shares issued (shares)	49,990,248	45,107,066
Common stock, shares outstanding (shares)	49,321,231	44,323,273