Mergers and Acquisitions - (Details)				9 Months Ended
	Dec. 31, 2018 location	Apr. 26, 2017 $ / shares shares	Jan. 03, 2017 USD ($) shares	Sep. 30, 2017 USD ($) location shares	Dec. 31, 2016 USD ($) shares	Sep. 30, 2016 USD ($) shares
Mergers and Acquisitions
Total assets				$ 11,169,110,000	$ 8,900,592,000	$ 8,797,165,000
Deposits				9,062,021,000	7,334,423,000	7,247,406,000
Loans				$ 8,223,204,000	$ 6,643,326,000	$ 6,489,068,000
Branches planned to be closed | location	2
Common Stock, Shares, Outstanding | shares				29,267,369	24,230,392	24,209,122
SBFC
Mergers and Acquisitions
Common stock shares issued | shares			4,978,338
Total purchase price			$ 435,115,000
Fixed stock exchange ratio			0.7307
Aggregate consideration			$ 434,609,000
Value of stock options			$ 490,000
PSTB
Mergers and Acquisitions
Common stock shares issued | shares		7,471,072
Fixed exchange ratio for shares issued (in shares) | $ / shares		$ 0.14
Total assets				$ 3,300,000,000
Deposits				2,500,000,000
Loans				$ 2,400,000,000
Number of full service branches | location				50
Branches planned to be closed | location	12