Consolidated Balance Sheets (Parenthetical) - USD ($) $ in Thousands	Jun. 30, 2017	Dec. 31, 2016	Jun. 30, 2016
Statement of Financial Position [Abstract]
Investment securities - held-to-maturity, fair value	$ 749,363	$ 784,172	$ 939,855
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
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	135,516,000	135,159,000	135,005,000
Common stock, shares outstanding	135,516,000	135,159,000	135,005,000