Mergers and acquisitions - Schedule of Consideration Paid and Allocation of Purchase Price to Net Assets Acquired (Details) - USD ($) $ / shares in Units, $ in Thousands		Jul. 31, 2017		Feb. 08, 2017	Jun. 30, 2018	Dec. 31, 2017	Jun. 30, 2017
Preliminary allocation of consideration paid:
Goodwill					$ 137,190	$ 137,190	$ 46,867
Total consideration paid		$ 236,484
Clayton Banks
Equity consideration
Common stock issued				1,521,200
Price per share as of July 31, 2017		$ 34.37
Total equity consideration		$ 52,284
Cash consideration		184,200	[1]	$ 184,200
Total consideration paid		236,484		$ 236,484
Preliminary allocation of consideration paid:
Fair value of net assets acquired including identifiable intangible assets	[2]	146,161
Goodwill		$ 90,323
Clayton Banks | Common Stock
Equity consideration
Common stock issued		1,521,200

[1] Amount was deposited into an interest-bearing account with the Bank in the name of the Seller as of July 31, 2017.

[2] Amounts include certain reclassifications of opening balances to conform to the Company’s presentation.