Subsequent Events (Details) $ / shares in Units, $ in Thousands			3 Months Ended			6 Months Ended
	Aug. 01, 2018 USD ($) $ / shares shares	Jun. 21, 2018 USD ($)	Sep. 30, 2018 USD ($)	Jun. 30, 2018 USD ($) branch $ / shares	Jun. 30, 2017 USD ($)	Jun. 30, 2018 USD ($) branch $ / shares	Jun. 30, 2017 USD ($)	Jul. 23, 2018 branch $ / shares	Dec. 31, 2017 $ / shares
Pending Acquisition of FCB Financial Holdings, Inc.
Common stock, par value (in dollars per share) | $ / shares				$ 1.00		$ 1.00			$ 1.00
Number of branches | branch				250		250
Redemption of Series C Preferred Stock
Cash dividends paid on preferred stock						$ 5,119	$ 5,119
Proceeds from issuance of stock						195,138	0
Dividends on preferred stock				$ 2,559	$ 2,559	$ 5,119	$ 5,119
Series D Preferred Stock
Redemption of Series C Preferred Stock
Proceeds from issuance of stock		$ 200,000
Subsequent Event | Series C Preferred Stock
Redemption of Series C Preferred Stock
Preferred stock, redemption price per share (in usd per share) | $ / shares	$ 25.00
Cash dividends paid on preferred stock	$ 2,600
Subsequent Event | Preferred Stock | Series C Preferred Stock
Redemption of Series C Preferred Stock
Stock redeemed during period (in shares) | shares	5,200,000
Stock redeemed during period	$ 130,000
Subsequent Event | FCB Financial Holdings, Inc.
Pending Acquisition of FCB Financial Holdings, Inc.
Common stock, par value (in dollars per share) | $ / shares								$ 0.001
Subsequent Event | FCB Financial Holdings, Inc.
Pending Acquisition of FCB Financial Holdings, Inc.
Number of branches | branch								50
Subsequent Event | FCB Financial Holdings, Inc. | Synovus Brand
Pending Acquisition of FCB Financial Holdings, Inc.
Common stock, par value (in dollars per share) | $ / shares								$ 1.00
Conversion ratio for common share								1.055
Scenario, Forecast | Subsequent Event | Preferred Stock | Series C Preferred Stock
Redemption of Series C Preferred Stock
Dividends on preferred stock			$ 4,100