Mergers and acquisitions - Schedule of Consideration Paid and Allocation of Purchase Price to Net Assets Acquired (Details) - USD ($) $ / shares in Units, $ in Thousands				9 Months Ended
	Jul. 31, 2017		Feb. 08, 2017	Sep. 30, 2017	Dec. 31, 2016	Sep. 30, 2016
Equity consideration
Total equity consideration				$ 52,284
Preliminary allocation of consideration paid:
Goodwill				$ 138,910	$ 46,867	$ 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	144,441
Goodwill	$ 92,043
Clayton Banks | Common Stock
Equity consideration
Common stock issued	1,521,200

[1] Amount was deposited into an interest-bearing account with the Bank of which $34,200 was included in cash and cash equivalents and interest-bearing deposits in the above schedule of net assets acquired as of July, 31, 2017