Business Combinations - fair value of acquired loans (Details) - USD ($) $ in Thousands	Dec. 31, 2016	Jun. 03, 2016	Mar. 11, 2016	Dec. 31, 2015	Jul. 17, 2015
Certain Loans Acquired in Transfer Not Accounted for as Debt Securities, Carrying Amount, Net	$ 453,686			$ 332,321
Bank shares, Inc [Member]
Certain Loans Acquired in Transfer Not Accounted for as Debt Securities, Carrying Amount, Net					$ 117,880
Certain Loans Acquired in Transfer Not Accounted for as Debt Securities, Acquired During Period, at Acquisition, at Fair Value					111,292
Bank shares, Inc [Member] | Commercial Real Estate [Member]
Certain Loans Acquired in Transfer Not Accounted for as Debt Securities, Carrying Amount, Net					82,782
Certain Loans Acquired in Transfer Not Accounted for as Debt Securities, Acquired During Period, at Acquisition, at Fair Value					81,191
Bank shares, Inc [Member] | Commercial Loan [Member]
Certain Loans Acquired in Transfer Not Accounted for as Debt Securities, Carrying Amount, Net					2,393
Certain Loans Acquired in Transfer Not Accounted for as Debt Securities, Acquired During Period, at Acquisition, at Fair Value					1,516
Bank shares, Inc [Member] | Single Family Residential Real Estate [Member]
Certain Loans Acquired in Transfer Not Accounted for as Debt Securities, Carrying Amount, Net					6,158
Certain Loans Acquired in Transfer Not Accounted for as Debt Securities, Acquired During Period, at Acquisition, at Fair Value					6,379
Bank shares, Inc [Member] | Construction Development Land [Member]
Certain Loans Acquired in Transfer Not Accounted for as Debt Securities, Carrying Amount, Net					979
Certain Loans Acquired in Transfer Not Accounted for as Debt Securities, Acquired During Period, at Acquisition, at Fair Value					913
Bank shares, Inc [Member] | Consumer And Other Loans [Member]
Certain Loans Acquired in Transfer Not Accounted for as Debt Securities, Carrying Amount, Net					14,575
Certain Loans Acquired in Transfer Not Accounted for as Debt Securities, Acquired During Period, at Acquisition, at Fair Value					13,692
Bank shares, Inc [Member] | Purchased Impaired Loans [Member]
Certain Loans Acquired in Transfer Not Accounted for as Debt Securities, Carrying Amount, Net					10,993
Certain Loans Acquired in Transfer Not Accounted for as Debt Securities, Acquired During Period, at Acquisition, at Fair Value					$ 7,601
Floridian Financial Group, Inc [Member]
Certain Loans Acquired in Transfer Not Accounted for as Debt Securities, Carrying Amount, Net			$ 280,022
Certain Loans Acquired in Transfer Not Accounted for as Debt Securities, Acquired During Period, at Acquisition, at Fair Value			266,137
Floridian Financial Group, Inc [Member] | Commercial Real Estate [Member]
Certain Loans Acquired in Transfer Not Accounted for as Debt Securities, Carrying Amount, Net			172,531
Certain Loans Acquired in Transfer Not Accounted for as Debt Securities, Acquired During Period, at Acquisition, at Fair Value			167,105
Floridian Financial Group, Inc [Member] | Commercial Loan [Member]
Certain Loans Acquired in Transfer Not Accounted for as Debt Securities, Carrying Amount, Net			39,070
Certain Loans Acquired in Transfer Not Accounted for as Debt Securities, Acquired During Period, at Acquisition, at Fair Value			37,804
Floridian Financial Group, Inc [Member] | Single Family Residential Real Estate [Member]
Certain Loans Acquired in Transfer Not Accounted for as Debt Securities, Carrying Amount, Net			38,304
Certain Loans Acquired in Transfer Not Accounted for as Debt Securities, Acquired During Period, at Acquisition, at Fair Value			37,367
Floridian Financial Group, Inc [Member] | Construction Development Land [Member]
Certain Loans Acquired in Transfer Not Accounted for as Debt Securities, Carrying Amount, Net			20,546
Certain Loans Acquired in Transfer Not Accounted for as Debt Securities, Acquired During Period, at Acquisition, at Fair Value			18,108
Floridian Financial Group, Inc [Member] | Consumer And Other Loans [Member]
Certain Loans Acquired in Transfer Not Accounted for as Debt Securities, Carrying Amount, Net			3,385
Certain Loans Acquired in Transfer Not Accounted for as Debt Securities, Acquired During Period, at Acquisition, at Fair Value			3,110
Floridian Financial Group, Inc [Member] | Purchased Impaired Loans [Member]
Certain Loans Acquired in Transfer Not Accounted for as Debt Securities, Acquired During Period, at Acquisition, at Fair Value			2,643
Floridian Financial Group, Inc [Member] | Purchased Credit Impaired [Member]
Certain Loans Acquired in Transfer Not Accounted for as Debt Securities, Carrying Amount, Net			6,186
Certain Loans Acquired in Transfer Not Accounted for as Debt Securities, Acquired During Period, at Acquisition, at Fair Value			$ 2,643
BMO Harris Bank [Member]
Certain Loans Acquired in Transfer Not Accounted for as Debt Securities, Carrying Amount, Net		$ 64,043
Certain Loans Acquired in Transfer Not Accounted for as Debt Securities, Acquired During Period, at Acquisition, at Fair Value		62,671
BMO Harris Bank [Member] | Commercial Real Estate [Member]
Certain Loans Acquired in Transfer Not Accounted for as Debt Securities, Carrying Amount, Net		31,564
Certain Loans Acquired in Transfer Not Accounted for as Debt Securities, Acquired During Period, at Acquisition, at Fair Value		31,200
BMO Harris Bank [Member] | Commercial Loan [Member]
Certain Loans Acquired in Transfer Not Accounted for as Debt Securities, Carrying Amount, Net		32,479
Certain Loans Acquired in Transfer Not Accounted for as Debt Securities, Acquired During Period, at Acquisition, at Fair Value		31,471
BMO Harris Bank [Member] | Purchased Credit Impaired [Member]
Certain Loans Acquired in Transfer Not Accounted for as Debt Securities, Carrying Amount, Net		0
Certain Loans Acquired in Transfer Not Accounted for as Debt Securities, Acquired During Period, at Acquisition, at Fair Value		$ 0