Consolidated Balance Sheets (Parenthetical) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017	Sep. 30, 2017
Investment securities - held-to-maturity, fair value	$ 505,566	$ 727,703	$ 740,564
Common stock, stated value	$ 1.00	$ 1.00	$ 1.00
Authorized and unissued common shares reserved for issuance	300,000,000	300,000,000	300,000,000
Common stock, shares issued	152,352,000	152,040,000	135,523,000
Common stock, shares outstanding	152,352,000	152,040,000	135,523,000
Series A Preferred Stock [Member]
Preferred stock, shares authorized	2,000,000	2,000,000	2,000,000
Preferred stock, shares issued	0	0	0
Preferred stock, shares outstanding	0	0	0