Consolidated Balance Sheets (Parenthetical) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017	Jun. 30, 2017
Investment securities - held-to-maturity, fair value	$ 524,597	$ 727,703	$ 749,363
Common stock, stated value	$ 1	$ 1	$ 1
Authorized and unissued common shares reserved for issuance	300,000,000	300,000,000	300,000,000
Common stock, shares issued	152,351,000	152,040,000	135,516,000
Common stock, shares outstanding	152,351,000	152,040,000	135,516,000
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