Portfolio loans - Allowance for Loan Loss (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Dec. 31, 2016
Changes in allowance for loan losses
Beginning balance	$ 49,201	$ 45,358	$ 47,795	$ 47,487
Provision for loan losses	1,494	1,950	2,494	4,050
Charged-off	(685)	(1,365)	(3,878)	(7,205)
Recoveries	1,025	1,904	4,624	3,515
Ending Balance	51,035	47,847	51,035	47,847
Amount allocated to:
Loans individually evaluated for impairment					$ 2,320	$ 3,467
Loans collectively evaluated for impairment					48,715	44,328
Ending Balance	49,201	45,358	47,795	47,487	51,035	47,795
Loans:
Loans individually evaluated for impairment					32,175	35,175
Loans collectively evaluated for impairment					5,041,432	3,842,183
Ending Balance					5,085,864	3,878,900
Purchased Credit Impaired Loans
Loans:
Loans collectively evaluated for impairment					12,257	1,542
Commercial
Changes in allowance for loan losses
Beginning balance	12,928	10,146	13,303	13,115
Provision for loan losses	336	1,502	(1,885)	2,747
Charged-off	(60)	(374)	(241)	(5,248)
Recoveries	318	92	2,345	752
Ending Balance	13,522	11,366	13,522	11,366
Amount allocated to:
Loans individually evaluated for impairment					1,477	1,535
Loans collectively evaluated for impairment					12,045	11,768
Ending Balance	12,928	10,146	13,303	13,115	13,522	13,303
Loans:
Loans individually evaluated for impairment					8,557	11,834
Loans collectively evaluated for impairment					1,310,889	947,993
Ending Balance					1,319,908	959,888
Commercial | Purchased Credit Impaired Loans
Loans:
Loans collectively evaluated for impairment					462	61
Commercial real estate
Changes in allowance for loan losses
Beginning balance	20,124	20,275	20,623	18,604
Provision for loan losses	418	(786)	1,477	1,110
Charged-off	(69)	(19)	(1,758)	(301)
Recoveries	403	37	534	94
Ending Balance	20,876	19,507	20,876	19,507
Amount allocated to:
Loans individually evaluated for impairment					818	1,778
Loans collectively evaluated for impairment					20,058	18,845
Ending Balance	20,124	20,275	20,623	18,604	20,876	20,623
Loans:
Loans individually evaluated for impairment					10,991	11,147
Loans collectively evaluated for impairment					2,195,464	1,642,010
Ending Balance					2,215,299	1,654,164
Commercial real estate | Purchased Credit Impaired Loans
Loans:
Loans collectively evaluated for impairment					8,844	1,007
Real estate construction
Changes in allowance for loan losses
Beginning balance	2,161	1,623	1,870	1,763
Provision for loan losses	64	212	1	(83)
Charged-off			(48)	(24)
Recoveries	36	169	438	348
Ending Balance	2,261	2,004	2,261	2,004
Amount allocated to:
Loans individually evaluated for impairment						11
Loans collectively evaluated for impairment					2,261	1,859
Ending Balance	2,161	1,623	1,870	1,763	2,261	1,870
Loans:
Loans individually evaluated for impairment					443	494
Loans collectively evaluated for impairment					246,232	181,584
Ending Balance					247,256	182,078
Real estate construction | Purchased Credit Impaired Loans
Loans:
Loans collectively evaluated for impairment					581
Retail real estate
Changes in allowance for loan losses
Beginning balance	13,681	12,979	11,648	13,714
Provision for loan losses	654	1,002	2,894	104
Charged-off	(482)	(860)	(1,574)	(1,305)
Recoveries	223	1,506	1,108	2,114
Ending Balance	14,076	14,627	14,076	14,627
Amount allocated to:
Loans individually evaluated for impairment					25	140
Loans collectively evaluated for impairment					14,051	11,508
Ending Balance	13,681	12,979	11,648	13,714	14,076	11,648
Loans:
Loans individually evaluated for impairment					12,139	11,644
Loans collectively evaluated for impairment					1,274,343	1,056,942
Ending Balance					1,288,852	1,069,060
Retail real estate | Purchased Credit Impaired Loans
Loans:
Loans collectively evaluated for impairment					2,370	474
Retail other
Changes in allowance for loan losses
Beginning balance	307	335	351	291
Provision for loan losses	22	20	7	172
Charged-off	(74)	(112)	(257)	(327)
Recoveries	45	100	199	207
Ending Balance	300	343	300	343
Amount allocated to:
Loans individually evaluated for impairment						3
Loans collectively evaluated for impairment					300	348
Ending Balance	$ 307	$ 335	$ 351	$ 291	300	351
Loans:
Loans individually evaluated for impairment					45	56
Loans collectively evaluated for impairment					14,504	13,654
Ending Balance					$ 14,549	$ 13,710